Compared to Telegram, it feels like using a laggy MSN Messenger. The experience, both client and server-wise, just feels very unpolished. It's no single big thing, it's more like death by a thousand cuts.
I was bullish on Matrix because it's so extensible, but in the end I realized that only the default client experience matters as that's the one everyone will be using. And it just isn't there yet. In the end, all the group chats I was in migrated to Discord or Telegram, so I had no more reason to use it...
We explicitly built Element X to be competitive with Telegram's UX - I'm guessing that the feedback here is on the crusty old Element Classic app, which hasn't been touched for 3 years now, and definitely did feel like a laggy MSN by comparison.
Meanwhile Element X feels really really good - especially on iOS, but also Android has improved loads in the last few months (after tweaking the rustc ARM compilation flags properly, doh)