It's good that we've converged on a standard approach for the core stuff - it's the associated functionality that's important and varies quite a lot.
Contact discovery and adding - Signal has gone with the WhatsApp approach here. There are downsides like needing a phone number but it makes it very easy and means that they can do things like sending you notifications when one of your pre-existing contacts joins Signal.
Backups - this is my main sore point with Signal, in terms of recommending it to my friends. Unless they're conscientious or put in extra work they're going to lose all their message history when they get a new phone. You need to do manual backup and restore to transfer messages, and if your phone is lost/stolen then you're screwed. I've set up sync but it's quite a lot of work for what (e.g.) WhatsApp does for free.