I use Signal heavily, mostly on desktop. It would be more accurate to say that Signal registration is mobile-only. Once registered, you can never touch your phone again and everything will work.

I've tried Telegram, WhatsApp, Hangouts, Facebook Messenger, iMessage. Signal has been the only one so far that I have been satisfied with because it:

* Supports end-to-end encryption (and "disappearing" messages)

* Has desktop and mobile apps that can be used independently (after registration at least; WhatsApp appears to require your phone to be on and near the desktop for the desktop app to work)

* Distributes messages to all Signal clients (Telegram was nearly unusable due to this problem - I had to mentally remember where the conversation was when switching between desktop and mobile)

* Doesn't have some sort of highly constrained user experience that requires using a social media platform or requires using a specific web browser (this has been a real problem with Hangouts and Facebook Messenger since they stopped supporting XMPP - RIP Adium)

* Supports mainstream desktop and mobile operating systems (iMessage fails at this, despite Apple's promises in the last decade)

* Has feature parity with other major chat services (this is the only weakness of Signal, but these days I can't think of a feature on WhatsApp or Telegram that isn't on Signal)

I agree that I wish they would steamline the registration so that you don't have to go through a somewhat convoluted process to get started, but it really is not a phone-only app. Desktop is a full first-class citizen after the initial fixed cost. I think that's the price they may pay by being a non-profit - maybe the marketing isn't exactly what Facebook/Google/Apple/<pick your own DataVortexCorp> can afford.

EDIT: Oh, I guess I should have mentioned Skype, but no thanks. Sorry, Microsoft, just no.