When I first looked at this, it boggled my mind that they require a phone number to sign up. Maybe that's no longer the case? But assuming it is, it just struck me as the epitome of breaking away from the concept of secure anonymity.
You really need to do research into this before making this claim. The phone numbers are hashed (Or something like it) before being sent to the Signal servers.
That's incorrect. Signal works perfectly fine with any mobile, landline or VoIP number.
Concerning multiple devices: I use Signal on my phone and on two desktops, works perfectly fine.
If that's the case, choosing to use an existing identity management system most people are part of is an excellent bootstrapping decision. Identity management is hard, and it's a fair problem to punt on if it's not your focus.
Encryption != anonymity. (In fact, it usually does the opposite, because encrypted messages stand out.)