If it’s anything like Beeper 's Matrix bridge then it’s E2EE Matrix encrypted between your device and the bridge server and then using Apple’s iMessage encryption between the bridge server and Apple/the other user.

The weak point is always going to be the bridge software as by necessity the message must be decrypted there to re-encrypt for iMessage.

At least in Beeper/Matrix the bridge software is open source and one can host their own bridge while continuing to use the existing Beeper/Matrix main server.

Doing so gives you no-trust security since the Beeper/Matrix host cannot decrypt the messages between you and the bridge you control and rubbing your own bridge eliminates that weak point.