Well, for a starter you will be limited to what email servers provide, which doesn't include video conferences, which most instant messaging solutions include these days

I have not noticed that problem, the messages are delayed by a few seconds, but not noticeably. Only using chatmail / own postfix though, YMMV.

Not in my experience. It feels like chatting on Signal or any other instant messenger, except there are no typing indicators, especially if you use DeltaChat's chatmail server (the default if you just say “let me in”).