I once implemented the presence and basic messaging functionality of XMPP for a web site, using a braindead bridge (that I also wrote; so, the meaningful logic lived in the browser).

Considering I'd never hosted an XMPP daemon and didn't know anything about the protocol (I'd used XMPP clients a little bit, but had never looked at the protocol) and got a server (that part, I didn't write), an auth connector for our website's authentication system (so the daemon would authenticate against that instead), prod-ready and the features I wanted all working smoothly and reliably in maybe three weeks of very part-time work (this'd be, like, 3-4 part-time days with LLMs now, tops, from the same starting point)... seems decent to me? I mean I did direct work with the protocol, didn't just glue together libraries, and it was pretty damn good. Also (and I know browsers seem to be retreating on this front, which sucks) being XML made it very nice to work with in a Web context, since you can just ask the browser to turn ~any XML into a DOM for you, and get a bunch of functionality for free.

What's wrong with it?

These days complicated and/or complex == hard == bad.