Earlier quoted context omitted.
I'm curious if you've actually used an IMAP client witha Gmail account; if so, which one(s). A sibling comment has already mentioned the folder/flags mixup issue, but while it sounds like one small feature, that one thing is so fundamentally broken that I really can't see how any user of Gmail IMAP would overlook it. I get duplicate notifications for the same mail in INBOX and [Gmail]/All Mail and actions on one copy…
I use Mail on OS X. I've never had any problems with Gmail. Folders work fine. I never use flags. Notifications work fine. When I delete a message, it's gone. Sometimes it takes a while for the deletion to propagate from one device to another, but I've never had a message survive deletion long term.
The gp was talking about RFCs and the IMAP protocol implementation itself, so I was talking about the Gmail server at the protocol level, rather than user interface.