There have been attempts by police where I live to exploit such setups as loopholes to request data from companies in foreign jurisdictions that they wouldn't be able to obtain from domestic ones, and since there is no "fruit of the poisonous tree" doctrine over here, such evidence has actually been admitted in court, or at the least used to enable parallel construction. Given that the Aussies are doubling down on their police state, I'd rather not expose myself to such shenanigans (and if my data ends up in some predictive policing data warehouse, I'd prefer if it was just one, not two).

As for advertisement surveillance, I'd have to take Fastmail's word, whereas with a domestic provider, there's the GDPR that is a lot harder to evade for them than for some Australian company.