Messages exchanged in Signal are repudiable. That said, in a recent blog post Signal indicated that arbitrary usernames is something they're working on.
> ...unlike XMPP you cannot spin up your own server and have full control over it.
Signal is a better alternative for the likes of PGP because it is centralized [0]. Spam notwithstanding, Matrix has made good of the federation model, however. So that's there too.
> And I am sure you cannot run an end-to-end audit of the whole Signal platform to verify that what they actually run...
Valid but shallow. Case in point: Public CAs, though held to accountability by browsers, are no where near as transparent as Signal is, and yet they form the backbone of almost all sensitive communication on the Interwebs.
> Signal is prone to censorship in those countries that decide to fight it.
This is the only major concern, but glad that Matrix exists, and so do other solutions. May be it isn't Signal's battle to fight. May be it is, and they'll figure something else out aside from building naive TLS proxies.