I don't agree with the claim that Signal has superior UX over Telegram, but it's definitely not insanely bad either. It has some rough edges, and it's desktop client is not that fully featured and doesn't look that great, but otherwise it's a perfectly fine app that my mother can use without problems and without specific instructions.

That's leagues above, say, Riot, or IRC, and I feel that "insanely bad UX" really paints the wrong picture.