The main complaints about Matrix I've heard though are about behind the scenes stuff rather than features, which the video touches on:

But there are some reasons why I think XMPP is superior. In Matrix, when you join a room, your server downloads and stores the entire history of that room. If someone on a federated server posts illegal content in a room you're in, your server is now hosting it, and you are liable. Whereas in XMPP, messages are relayed in real time. Group chat, MU history stays on your server hosting that room. So your server only stores messages for your users which means that no content caching there is no content caching from other servers. This is a fundamental architectural difference which makes the XMPP protocol better in my opinion.

Personally I don't know that much about it but I briefly looked into what it would take to write a client for Matrix a few years ago and it seemed pretty daunting to work with. Maybe it would be possible to write software that implements more Discord features on top of XMPP to have something that works more smoothly.