I don't think people are only concerned about the government, but rather the corporations that own your data (via storing the encryption key to use whenever they want, to look through whatever they want)
In the end signal and imessage (with backups off) provide roughly the same claimed guarantee here (on a technical level at least). They both promise not to secretly inject additional client keys into your account, (to at least obtain a forward-operating 'wire tap'), but the software you're running is all opaque and could certainly do so if they wanted (or were compelled) to.