I’m looking into hosting one of these for the first time. From my limited research, XMPP seems to win in every way, which makes me think I must be missing something. Matrix is almost always mentioned as the de-facto standard, but I rarely saw arguments why it is better than XMPP?
Xmpp seems way easier to host, requiring less resources, has many more options for clients, and is simpler and thus easier to manage and reason about when something goes wrong.
So what’s the deal?
Yes, Monal is catching up to Siskin fast, but until recently didn’t support a/v calls which is why many people still prefer Siskin.
As for a unified system, have a look at https://snikket.org which offers a one-stop solution under a single brand similar to Element. It uses lightly modified versions of Conversations, Siskin and Prosody (as a server) under the hood.