There are more than 370 XEPs.
Here's the first one I found that causes server<->server incompatibility: https://xmpp.org/extensions/attic/xep-0369-0.7.1.html
What was always missing was a golden set of standards. Maybe XMPP "2" is XMPP and a set of XEPs and so on.
Otherwise all you have is a bunch of half-working XMPP implementations.
Very famous ones of course include voice/video. The UX on that is atrocious.