It's trivial to block several distributed hosts simultaneously. An aspiring censor would simply find the most common federated endpoints for a given service and block all of them. Only the users of that software would be affected. There wouldn't be any collateral damage.

If the censors somehow didn't hit every single worthwhile federated endpoint, users would still be left wondering why they couldn't communicate with most of their friends. Moving between federated hosts would also necessitate an entirely new identifier, so users would need to rebuild their social graph again.

In addition to being ineffective against censorship, there are several other properties and trade-offs that make federation a difficult proposition for an application like Signal: https://signal.org/blog/the-ecosystem-is-moving/