Oh, there could be a massive list here, but as tiny exemplar - try zooming in to see something in the corner of an image shared while you're on a video call. You could, before they invented "Huddles", an even shittier version of the video calls feature it had before, but now you can't.

I guess you also haven't been hit yet by the "every message is a new message even if I'm the one that just sent it and it's right there on screen in the channel I still have open because I sent it" bug yet. When you do, I'm afraid the only fix is to individually sign out of every Slack you're in, delete the app, delete every file it's ever created in ~/Library/, reinstall the app, re-sign-in to every Slack again.