Search sucks: really. unable to find stuff unless you know the sender, their full text search seems to only work in a browser and only after indexing (which can take an hour) and it will go away with time as it's browser local storage.
Sending emails directly from share sheet in iOS failed twice.
Unable to add an invite to a calendar invite on iOS or in the Webapp (if invited by someone else).
Emails broken format when forwarding to someone else.
Long delay in delivering a simple email (to google and apple) at least three times.
The way that the emails are threaded. And that trash still shows as being in the same thread, instead of moving apart.
False sense of security as most emails are only encrypted on one side. And mine are visible on the other side.