Their USP is "security made simple", in other words: use Signal because it's as easy as the mainstream alternatives, but far more secure (but not perfect).
That only works if they're actually more secure than the mainstream alternatives. I don't think they are; Signal has transport encryption but the system probably isn't secure against Open Whisper Systems themselves or someone who controls the central server, which is the same security situation you'd be in with any of the alternatives I mentioned.