I will focus on the things that people rarely point out. Btw. to be clear, I largely use Linux personally and professionally. So no evangelism here.
First of all: The BSDs are ALL general purpose operating systems, not distributions. I just mention this because there is that general "OpenBSD is for routers" and "NetBSD is for old hardware" nonsense. To give you an example. Various things including 3D graphics acceleration for various devices has been available in OpenBSD and DragonFly a lot earlier than in eg. FreeBSD even though FreeBSD for example has proprietary NVIDIA drivers available unlike the rest. People use all of them on server, as desktops and even on gaming devices. It's even kind of a myth in BSD communities that "this BSD is for X".
Something that people who aren't in the BSD users already and especially ones that do use Linux sometimes misunderstand is that the BSDs are independent of one another. Eg. OpenBSD and FreeBSD are as different as eg. FreeBSD and macOS, which is also a BSD and has A LOT of kernel and user space level code shared with FreeBSD. That's why grep -v says FreeBSD that's why docker uses (or used) xhyve which is FreeBSDs bhyve for macOS, which is why you got kqueue and dtrace, etc.
So in that respect "the BSDs" doesn't really work that well and is "dangerously" oversimplifying things.
All of them have a pretty good security track with strengths and weaknesses compared to each other and Linux. I would say it's overall even. But depending whom you ask the difference may very, because even in the Linux world there are different approaches to security. And as anyone having anything serious to do with security it's not that easy to measure security in first place. OpenBSD has a strong focus and achieves a lot on the C side and people who don't use any BSD at all actually and basically don't know it have critical C code compile on OpenBSD to spot issues affecting their actual target platform.
As for development. All of the BSDs increase in terms of development (only happen to know it cause someone once pointed me at an obviously wrong graph). I got a bit interested in that comparison because something that people like to underestimate is that single individuals are still the ones doing huge chunks of big projects. It's insane. Linux kernel might be one of the very few exceptions because a huge amount of people make hardware essentially for it for a living (SBCs and so on). Even if you look at other "big" projects like Postgres and so on which huge companies absolutely rely on it's a couple of people doing insane amounts of work. You have that in all BSDs.
FreeBSD sees a lot of usage as a base for commercial OSs (video game consoles, Netflix cache servers, etc.). However to be fair while some parts come back to the OS they also have companies that shamelessly take code without giving pretty much anything back (looking at you, Apple). Nobody is angry at that cause that's the terms and people in the BSDs often contribute out of the drive to make a great OS.
Something in my opinion greatly overlooked is that FreeBSD has one of the best package bases of all open source operating systems including all the Linux distributions. This is somehow rarely advertised but you have a huge official, very stable, very up to date, highly configurable "repository". I think this is pretty much unmatched elsewhere. It's like having Arch AUR, but without the security nightmare nor instability. In fact getting a commit bit is on the harder side compared to many Linux distributions. Many official package maintainers cannot commit on their own. This means that the "many eyes" does a lot of work there.
You get binary packages with one of the best binary package managers out there (classic case of being late to the game, but taking all the leanings and doing a great implementation).
You get close to 35k of these "ports" both binary or compiled if you want. If you compile yourself you get a huge amount of flexibility. My go to example for this is pick and choose a Postgres and PostGIS version combination, but there are many other examples from disabling adding docs, to optimizations to a stunning amount of third party nginx patches giving you better than commercial version experience. That coupled with a great flexible automatic build tool called poudriere, which you can use to create repositories of all or parts of these 35k ports and your chosen options for different versions of FreeBSD.
I am not completely sure but the sheer amounted of vetted(!) packages is unseen anywhere else. There is a security database, there is deprecation warnings and since the base OS and package separation (maybe the common differentiator of BSDs compared to Linux) you can match stability with the latest packages (or older version, eg. in Postgres), which is something I really miss in the Linux world even though there is nothing that is because of the OS/kernel it runs. It's simply nobody in the Linux world did put the effort in so far.
While FreeBSD has had a couple of not so great defaults - I think maybe to do with how many long term FreeBSD users any ways have their custom configs and trying to be compatible with upgrades - I think it got better. On the other hand they have and had a lot of projects on that front. jails predated pretty much everything else in the open source world, there was the TrustedBSD project, now there is CherBSD which puts a capabilities system in to hardware, etc.
OpenBSD also has a strength that is overlooked in that for a lot of "set and forget" style applications the base OS is enough. You get a highly secure (custom) HTTP server, a custom acme client, a custom load balancer (relayd), one of the best firewall implementations, etc. out of the box. I love to use it for simple I need something so that one can upload files via SFTP to a HTTP server setups. Just run syspatch and every half year sysupgrade and you are good. OpenSMTPD is also an absolute breeze and also having a Go application running with pledge and unveil (basically a way simpler easy to use seccomp) is great. Also for some reason there is a serious gaming community on OpenBSD making things work without wine (doesn't even work on the OS) in a way that is hard to believe.
Another thing is that a lot of "everyone uses them" tools come out of OpenBSD circles. Eg. tmux, OpenSSH, sudo (even though doas replaces it), OpenNTPD, OpenBGPD femail, etc. Also LibreSSL and sndio which always pop up on Linux in the least expected places (like games) for some reason. They all of that heavy focus on simplicity, but done in a non-academical but very real world scenario. They also do a lot of work in the compiler scene on the security side.
NetBSD and DragonFly also have their often very unexpected strength. Eg. DragonFly supports multiple ways of full file system encryption, including truecrypt compatible one. NetBSD while working on a Dreamcast also has very interesting and it runs in unexpected places like the ISS and somehow there have always been DARPA contributions, while in all BSDs at least subjectively the most to NetBSD. Mostly because there I've also seen quite a few pkgsrc (their non-base-OS packages) contributions. pkgsrc probably is one of NetBSDs biggest impacts being using by lots of other OSs from Minix to illumos. And on that topic, Android has (had?) quite a few NetBSD parts (in place of GNU userspace software) and Microsoft took the network stack from the BSDs and Internix (which was the predocessor to the Linux subsystem from when they wanted to be compatible with Unix datacenters, etc.) was largely a modified OpenBSD system.
Again, not a BSD users, and BSD people being very practical and less of a cult they'll usually tell you to keep using Linux if it works for you. But I think there also are a lot of misinterpretations when you used Linux all your life.
In fact the hardest thing on just trying it will probably be it NOT being Linux and also when switching between BSDs. They are simply other approaches to making an operating system. So if you take away one thing from this it's they are general purpose operating systems in their own respect and while they share code so does macOS, which as mentioned is also a BSD.
Use Linux if it works for you, but just for people trying it out: If you wanna actually be able to compare it you really have to use it and get it else you will just be annoyed with it not being Linux. Kind of like when you try a different editor, programming language, etc. Just don't make the mistake thinking BSDs are like distributions. That's a bit like thinking AIX and macOS are just Unix distributions. Even Solaris I think in very old versions was BSD based. ;)
First of all: The BSDs are ALL general purpose operating systems, not distributions. I just mention this because there is that general "OpenBSD is for routers" and "NetBSD is for old hardware" nonsense. To give you an example. Various things including 3D graphics acceleration for various devices has been available in OpenBSD and DragonFly a lot earlier than in eg. FreeBSD even though FreeBSD for example has proprietary NVIDIA drivers available unlike the rest. People use all of them on server, as desktops and even on gaming devices. It's even kind of a myth in BSD communities that "this BSD is for X".
Something that people who aren't in the BSD users already and especially ones that do use Linux sometimes misunderstand is that the BSDs are independent of one another. Eg. OpenBSD and FreeBSD are as different as eg. FreeBSD and macOS, which is also a BSD and has A LOT of kernel and user space level code shared with FreeBSD. That's why grep -v says FreeBSD that's why docker uses (or used) xhyve which is FreeBSDs bhyve for macOS, which is why you got kqueue and dtrace, etc.
So in that respect "the BSDs" doesn't really work that well and is "dangerously" oversimplifying things.
All of them have a pretty good security track with strengths and weaknesses compared to each other and Linux. I would say it's overall even. But depending whom you ask the difference may very, because even in the Linux world there are different approaches to security. And as anyone having anything serious to do with security it's not that easy to measure security in first place. OpenBSD has a strong focus and achieves a lot on the C side and people who don't use any BSD at all actually and basically don't know it have critical C code compile on OpenBSD to spot issues affecting their actual target platform.
As for development. All of the BSDs increase in terms of development (only happen to know it cause someone once pointed me at an obviously wrong graph). I got a bit interested in that comparison because something that people like to underestimate is that single individuals are still the ones doing huge chunks of big projects. It's insane. Linux kernel might be one of the very few exceptions because a huge amount of people make hardware essentially for it for a living (SBCs and so on). Even if you look at other "big" projects like Postgres and so on which huge companies absolutely rely on it's a couple of people doing insane amounts of work. You have that in all BSDs.
FreeBSD sees a lot of usage as a base for commercial OSs (video game consoles, Netflix cache servers, etc.). However to be fair while some parts come back to the OS they also have companies that shamelessly take code without giving pretty much anything back (looking at you, Apple). Nobody is angry at that cause that's the terms and people in the BSDs often contribute out of the drive to make a great OS.
Something in my opinion greatly overlooked is that FreeBSD has one of the best package bases of all open source operating systems including all the Linux distributions. This is somehow rarely advertised but you have a huge official, very stable, very up to date, highly configurable "repository". I think this is pretty much unmatched elsewhere. It's like having Arch AUR, but without the security nightmare nor instability. In fact getting a commit bit is on the harder side compared to many Linux distributions. Many official package maintainers cannot commit on their own. This means that the "many eyes" does a lot of work there.
You get binary packages with one of the best binary package managers out there (classic case of being late to the game, but taking all the leanings and doing a great implementation).
You get close to 35k of these "ports" both binary or compiled if you want. If you compile yourself you get a huge amount of flexibility. My go to example for this is pick and choose a Postgres and PostGIS version combination, but there are many other examples from disabling adding docs, to optimizations to a stunning amount of third party nginx patches giving you better than commercial version experience. That coupled with a great flexible automatic build tool called poudriere, which you can use to create repositories of all or parts of these 35k ports and your chosen options for different versions of FreeBSD.
I am not completely sure but the sheer amounted of vetted(!) packages is unseen anywhere else. There is a security database, there is deprecation warnings and since the base OS and package separation (maybe the common differentiator of BSDs compared to Linux) you can match stability with the latest packages (or older version, eg. in Postgres), which is something I really miss in the Linux world even though there is nothing that is because of the OS/kernel it runs. It's simply nobody in the Linux world did put the effort in so far.
While FreeBSD has had a couple of not so great defaults - I think maybe to do with how many long term FreeBSD users any ways have their custom configs and trying to be compatible with upgrades - I think it got better. On the other hand they have and had a lot of projects on that front. jails predated pretty much everything else in the open source world, there was the TrustedBSD project, now there is CherBSD which puts a capabilities system in to hardware, etc.
OpenBSD also has a strength that is overlooked in that for a lot of "set and forget" style applications the base OS is enough. You get a highly secure (custom) HTTP server, a custom acme client, a custom load balancer (relayd), one of the best firewall implementations, etc. out of the box. I love to use it for simple I need something so that one can upload files via SFTP to a HTTP server setups. Just run syspatch and every half year sysupgrade and you are good. OpenSMTPD is also an absolute breeze and also having a Go application running with pledge and unveil (basically a way simpler easy to use seccomp) is great. Also for some reason there is a serious gaming community on OpenBSD making things work without wine (doesn't even work on the OS) in a way that is hard to believe.
Another thing is that a lot of "everyone uses them" tools come out of OpenBSD circles. Eg. tmux, OpenSSH, sudo (even though doas replaces it), OpenNTPD, OpenBGPD femail, etc. Also LibreSSL and sndio which always pop up on Linux in the least expected places (like games) for some reason. They all of that heavy focus on simplicity, but done in a non-academical but very real world scenario. They also do a lot of work in the compiler scene on the security side.
NetBSD and DragonFly also have their often very unexpected strength. Eg. DragonFly supports multiple ways of full file system encryption, including truecrypt compatible one. NetBSD while working on a Dreamcast also has very interesting and it runs in unexpected places like the ISS and somehow there have always been DARPA contributions, while in all BSDs at least subjectively the most to NetBSD. Mostly because there I've also seen quite a few pkgsrc (their non-base-OS packages) contributions. pkgsrc probably is one of NetBSDs biggest impacts being using by lots of other OSs from Minix to illumos. And on that topic, Android has (had?) quite a few NetBSD parts (in place of GNU userspace software) and Microsoft took the network stack from the BSDs and Internix (which was the predocessor to the Linux subsystem from when they wanted to be compatible with Unix datacenters, etc.) was largely a modified OpenBSD system.
Again, not a BSD users, and BSD people being very practical and less of a cult they'll usually tell you to keep using Linux if it works for you. But I think there also are a lot of misinterpretations when you used Linux all your life.
In fact the hardest thing on just trying it will probably be it NOT being Linux and also when switching between BSDs. They are simply other approaches to making an operating system. So if you take away one thing from this it's they are general purpose operating systems in their own respect and while they share code so does macOS, which as mentioned is also a BSD.
Use Linux if it works for you, but just for people trying it out: If you wanna actually be able to compare it you really have to use it and get it else you will just be annoyed with it not being Linux. Kind of like when you try a different editor, programming language, etc. Just don't make the mistake thinking BSDs are like distributions. That's a bit like thinking AIX and macOS are just Unix distributions. Even Solaris I think in very old versions was BSD based. ;)