If they ever chose to implement remote attestation for bridges, and you chose to use an open source matrix client to control your local keys, there is at least a potential path for you to prove they can't look at messages so you don't have to trust them.
You also have the option to self host bridges and take your ball and go home at any time, given that Matrix is an open protocol. Minimal lock-in.
Texts.com by contrast is completely proprietary as far as I can tell, with no path to self host, so it is strictly worse than Beeper in every way in terms of transparency.