> Two XMPP clients might support completely different feature sets.

This is of course true of Matrix as well. Just because you document everything in one place doesn't mean every app will support it all. Or that every app will even want to support it all. If every app were exactly the same there would be no point in having multiple apps, after all.

> The network effects of WhatsApp/Signal/iMessage are the real enemy, not protocol design.

Absolutely.