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.
This is untrue. By design, messages are never decrypted on servers when end-to-end encryption is in use. They would have to break the encryption first, because they don’t have the keys.
I assume you also have to trust the servers which the accounts you’re messaging are stored on. (Although there are real situations where all users will be on the same server, where this is obviously a great benefit.)
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.
Isn’t Signal E2E encrypted? How would it be able to decrypt them?
This is untrue. By design, messages are never decrypted on servers when end-to-end encryption is in use. They would have to break the encryption first, because they don’t have the keys.
I assume you also have to trust the servers which the accounts you’re messaging are stored on. (Although there are real situations where all users will be on the same server, where this is obviously a great benefit.)