Aside from the closed JMAP push API, my only complaint is that the spam folder doesn't work as you'd expect in IMAP. Specifically, moving a message to the spam

Aside from the closed JMAP push API, my only complaint is that the spam folder doesn't work as you'd expect in IMAP. Specifically, moving a message to the spam folder doesn't influence the spam detection algorithm, as it does in the Web interface. To achieve this in IMAP, you must create a 2nd spam folder that is specifically configured to behave this way, which is really stupid.