The downside to Telegram? I don't trust it - so even if any of the above we're true they're all subjective and in my mind not worth compromise.
As much as I hate fb, messenger is a pretty decent application.
I tried it with my friend some time ago and we just couldn't figure out in a reasonable amount of time how to add each other etc.
I consider both of us tech savvy and we have absolutely no trouble with other IM clients or more archaic stuff like IRC.
As I stated our entire family uses Signal across Android, IOS and desktop. That age range in our entire family group is 20s to 70s and I happen to be the only user who would be considered tech savvy.
We didn't have each others phone numbers in our contacts.
Manually adding by phone number didn't work.
At this point we just gave up.
I use prepaids because I travel a lot so my mobile numbers are just throwaways.
I'd rather not give _all_ of my contacts to an app anyway.
So there really should be a way to manage contacts inside the Signal client and add people via screen names or email addresses.
I agree that it would be nice to have another root artifact for using signal (screen name, email, etc). The one frustrating piece about Signal is not being able to use it across mobile devices.
However it needs to be considered Signal was originally securing texts via TextSecure. It didn't, fundamentally, come up as a Slack or Telegram competitor. So that design decision may make it feel "behind" in comparison. However, I'll again state that I don't trust the others. There's no way you could force me to use FB Messenger or Google (product here). Most complaints here are fundamentally still UI/UX, not privacy. And useable privacy was the original intent. Signal has that in spades.
I'm curious though; what happens if someone takes over your phone number? Eg. If the operator re-assigns a prepaid number?