I'm guessing you created an account on the default Matrix.org homeserver? While hardware requirements have been steadily dropping for months now, the default seems to attract more than enough people to negate these advances. So while Matrix.org accounts and rooms may feel slow, I hope you'll be willing to give it another shot some day in the future. To that end, I would highly recommend setting up your own server (as the French and German government agencies have done), or even hopping on mine (email is on my profile) if you just want to test it for a day or two.

As far as the cumbersome UI/UX - you got me. While some people like it, many people seem to disagree. Over time, the myriad of clients will presumably get better, and hopefully the people not currently enamored with the UX can find one they like :)

Regarding reliability, you may have to expand on that. I've been using my Matrix server (and Telegram / Whatsapp bridges) to chat for months now, and have had messages come through faster for me than others in the same group chat!

All that said, maybe you just had a bad experience. I'd never try and say you didn't, but hopefully there was another explanation than just "Matrix is still nowhere close to ready".