“Signal is being blocked in Venezuela and Russia. The app is a popular choice for encrypted messaging and people trying to avoid government censorship, and the blocks appear to be part of a crackdown on internal dissent in both countries…”
It doesn’t matter if it is US based. You shouldn’t trust the server.
Signal has known issues. That doesn’t mean it is entirely bad though. Saying things like Signal is insecure is simply untrue. It has weaknesses but it also has the benefit of protecting your messages completely and being well established.