* has (almost) no tests,
* has no CI (at least I could find none),
* lacks some basic features competitors have,
* sometimes doesn't ring when you call, and
* they just closed all 1100 open issues without reading their contents [1]."
Don't get me wrong, I think Signal is a great idea and have contributed some code to it, but these look like some red flags to me.
Obviously hard to compare to Telegram, which is closed source so we can't know whether they have similar problems.
[1]: https://github.com/signalapp/Signal-Android/issues/7598