Where the metadata goes I think is important as well.
All Signal metadata necessarily goes through Signal’s servers and is tied to your phone number, but not all Matrix metadata ever gets near the Matrix.org if you are using a different homeserver.
I think both are less than ideal in that regard, and I think Briar (strictly P2P) has a much better model for dealing with this at the expense of generally being a UX disaster.
The server software appears to be available and updated now, which they’ve been spotty about in the past. I’ve updated to remove the closed-source part since that is not correct.
As for phone number: Signal still requires me to enter a phone number to create an account as of about 5 minutes ago.
Signal is centralized, closed-source, not-selfhostable (edit: in any meaningful way) and requires being attached to a phone number. (Edit: server source is available, but self-hosting requires recompiling and distributing a custom app to all of your contacts to actually use it.)
Matrix is decentralized, federated, fully open source with multiple client and server implementations, self-hostable, and does not require being attached to a phone number.
Given how common it is for people to use the ‘reset password’ link for this exact purpose, it does make it seem kinda redundant to even implement passwords on many services to begin with.
Solid Explorer