Security is not a boolean, but when your whole selling point is security you'd better be good at it.
I trust Signal the same amount as Facebook Messenger or any other centralised messaging system that uses transport encryption (e.g. Skype, IRC+SSL, WhatsApp...). But what's the USP that means I should use Signal rather than any alternative?
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.
Not giving all your metadata to Facebook (Messenger, WhatsApp) or Microsoft (Skype) for a start. The message may be e2e encrypted, but damn sure these companies are logging as much as possible about who's chatting with whom, when, & where each of them are, how much data is sent each time, etc.
Some people trust Moxie / OWS more than the others, and with good reason.
I trust Signal the same amount as Facebook Messenger or any other centralised messaging system that uses transport encryption (e.g. Skype, IRC+SSL, WhatsApp...). But what's the USP that means I should use Signal rather than any alternative?