Comment by einpoklum

Signal is quite likely to fail on account being a single organizational entity running the service and refusing to adapt their protocol for federation. In fact, it might already have happened (partially) and you wouldn't know, because they're US-based. The US has the National Security Letters mechanism:

https://en.wikipedia.org/wiki/National_security_letter

the government can compel entities to let them access their systems and keep it a secret.

Also, if Signal runs on AWS, that is another potential vector of surveillance.