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.
“What is your favorite self-hosted application?” had what looks to be about 15 matrix responses.
Would potentially be interesting to see Matrix/XMPP/etc prevalence in future surveys, maybe replacing ‘what activitypub apps’ with a more generic ‘what federated apps do you self-host’