Matrix is decentralized ( same as Xmpp ), but unlike Xmpp, over 90% of all users are on same server ( Matrix server, which is btw using Amazon Aws ). Also, very big majority of users is using the same client, Element.
In practice I don't see much advantages compared to Xmpp, or to be precise, if we compare it to Xmpp + Omemo, I don't see even one advantage in favor of Matrix.
Xmpp have real decentralization hundreeds of servers, multiple different clients with different features, not one or another being dominant.
There are other, not decentralized, but really P2P and/or blockchain based e2ee chat platforms that should be taken in consideratiom
Briar being the number one among them
Tor based, serverless, opensource, have local "internetless", mesh, capabilities
Berty ( Ipfs based, serverless, opensource, but still in beta version ), also haves local mesh capabilities like Briar
Adamant ( I think first blockchain based chat app, which requires you to buy their useless coin, but from opsec perspextive is very good )
Status Im ( based on Ethereum blockchain, open source )
Ricochet Refresh
P2P tor based chat
Speak
Fork of Ricochet Refresh, also having a mobile client
Altough mobile clients isolation is not nearly as good as Briar's
Cwtch
Similar to Briar and Ricochet
Opensource, P2P, Tor based
Supports multiple accounts per device, multiplatform
Still very slow
