Yeah, hearing TG's UI/UX described as not good made me do a double take. It's the best UI I've ever seen in a chat app, and I've probably used at least a dozen by now.
I find the UI/UX of Signal to be good enough that I used it as my default messaging app, even though I only have a few contacts who use(d) Signal.
It took until Oreo/Pie for the stock messaging app to become equally good or better, and by then none of my contacts bothered with using Signal anymore. It's now 0% Signal, 0.1% text messages, 0.1% WhatsApp, 99.8% FB Messenger, which is really depressing.