>>> You're still only as secure as the client on the other side of the conversation. If that one is compromised ... it could very well be sending all messages in clear text to a malicious party.

>> There's no technical solution in any technology for preventing the other side being compromised, as far as I can see.

I don't know Matrix, but I can guarantee that it doesn't solve the problem of a compromised client obtaining the messages willingly sent to it.