The biggest issue I have with Fastmail is how lousy the tech support is. I had issues with IMAP and they take days between replies to get back with you.
I started sending them hourly updates hoping for a faster response. Then I copied the CEO to the email chain. After about 20 emails to them I gave up.
A few days later someone got back to me that every time you email their support it resets their clock on when they will respond...
Literally the worst of the worst in email support. That is sad because they have good policies and a fairly robust web frontend.