I'm simply talking about scenarios where one API needs to talk to another to get a resource it needs to act upon in some way. This can get vary hairy very quickly especially when each microservice ends up with copies of resources owned by other microservices that it now has to update asynchronously via pub/sub any time there is a change. This was the biggest challenge. And I think any teams willing to switch to this type of architecture need to look at it from this angle because monitoring and troubleshooting such systems is quite an undertaking.