I'm interested: aside from the phone number/backup issues well documented on every HN thread about Signal, which features do you think that Signal lacks?
It's important to realize that Signal is aiming to be iMessage replacement Whatsapp/Telegram/Discord are more like "chatservers" with rooms. Signal right now focuses on simple 1to1 messaging with sending pictures/media etc. That's what they do really well.
Btw iMessage also have super basic desktop client. It works even worse than Signal desktop (random logouts and messages sent as different account with only "email"). It might not be so easy going from 1v1 fully encrypted chat to manytomany fully encrypted chatroom megaserver. Apple seems to also struggle with it.