You can’t know with certainty on Signal that the client and the server are actually keeping your messages encrypted at rest, you have to trust them.

With Matrix, if you self host, you are the one in control.

Decentralization actually can be really powerful to give you a backup even if you prefer Signal; Signal’s servers very infrequently go down, but when they do, you entirely lose that channel for an unpredictable amount of time.

That is certainly an improvement over Signal, yeah.