The first thing you see when you go to twitch.tv is a live stream and a list of other live streams.

Once you view a stream the only things you see are the video player, details about the streamer, and chat.

Not sure what you imagine could be better, but if you want to know what really bad UI looks like, check out Mixer.

At least on twitch I'm able to watch somebody's video if I wanted to. Which seems like a very basic functionality you'd expect from a video website.

That's because it's not really a "video website", it's more like a patreon for video creators.

Floatplane has made a very conscious decision not to influence the user's decisions on consuming content; there's no algorithm, recommended videos, or trending page. you just have access to the videos of people you want to support.

Twitch doesn't look unpolished though. I don't like it, but its flaw isn't being unpolished.