Middle is Matrix and ActivityPub, right is Session, SimpleX, and Nostr

Matrix has de-facto centralization around Matrix.org where all metadata floats back through them in one way or another. Due to the expense of all the mirroring required (every message + attachment from every DM & chatroom of every user) as well as the Python back-end being slow, it’s in practice not good to self-host despite in theory Matrix being good. XMPP runs on a potato relatively & has so much feature overlap (with the ability to extend being inherent to the protocol)—the only issue is fragmentation of servers & clients (though Conversations Compliance is close-ish to a minimal standard).