IMHO, aspects like metadata and application security are often much more important than the cryptographic protocol when assessing the security of a system like a messenger. People e.g. love to celebrate the Signal protocol because of its double-ratchet key rotation scheme, which is of course great and provides a little bit of additional security, but the other technical and organizational aspects of the system are just as important to assure privacy and security.

From that angle I like systems like Matrix way more than centralized solutions like Signal, Threema etc., because the whole security and privacy guarantees of the latter can be completely nullified by a single software update. If users don't have full control over the software running on the endpoint then end to end encryption is little more than a marketing gag, IMHO.