Fastmail is fantastic, no complaints at all.... besides the mobile app. I don't care about the speed or glitches or whatever others may complain about, but I travel way too much to have no offline email access on my phone. So this turns into me having 2 email apps on my phone (Android); one to do stuff in (Fastmail) and another (K-9 which is not great to write/do stuff in) that just sits there, likely hogging battery…
Just wondering: why don't you use your phone's native email app and IMAP?
Another reason is the lack of push notification (not sure if i'm using the right term) when using the built in gmail app for fastmail. It only gives me the option to poll every 15 minutes. The gmail app also doesn't support actions like snooze, swipe to archive etc on non-gmail accounts and I like having that in the fast mail app.