Do all video apps use WebRTC now? Then why does call quality vary so much?

Signal's blog post about video calling caught my attention today. Moxie says they switched to webRTC for the actual streaming part of the call. That's what Hangouts, Mozilla Hello, Whatsapp, and Skype use, too.

So why does it seem like call quality varies so much between services? Is it just different logic for codec/size negotiation? If that's the case, why is (non-webRTC, closed source) Facetime so much more reliable than the rest?