XMPP is not good. The clients are poor. The core server requires a bunch of extensions to do basic stuff like sync history with clients. I have my gripes with Matrix but I ran right back to it after trying xmpp for a month. Yes I tried the distros etc etc, the stack is too old and people aren't modernizing it.

I don't understand why people keep bringing it up as if it is a viable modern replacement for Matrix, it isn't.