Just off the top of my head: on iOS, the app is frequently slow to download new messages, occasionally (but not frequently) crashes or freezes up, opening a message from a notification is unreliable, it doesn’t support landscape mode, the search feature sucks (no filtering, sorting, etc), and it has some questionable design choices. Like, why does it include spam in the “All Mail” category? And why is it that swiping a message right sends it to the trash when doing that exact same interaction in the iOS mail app marks it as read? I’ve adapted to the difference after all these years, but it’s clearly a bad design.
Overall it’s not terrible; I’d give it like 4/5 or 3.5/5 stars, however with the price I’m paying for this (IMO overpriced) service I’d expect something a little better. I will say that the experience today is much better than it was a year ago, so even though it takes a long time, it does seem to be improving.