For the UI it's been a minute since I played but I think mostly all of it. For the platforming, I like difficult platformers- Celeste and melee are 2 of my favorite games (obviously melee isn't exactly the same genre but still a very precise platformer). I thought the type of difficulty in certain parts wasn't the "fun" kind of difficulty. I don't mean that in an insulting way and I don't think it was an issue with all of the platforming challenges in the demo by any means. Idk how to describe it exactly but I think that specifically the difficult parts that I didn't like were where I had to time a jump at a very precise spacing before falling off of a platform combined with the fact that the checkpoints were pretty infrequent. I think it's okay to have very difficult jump like that, but it's frustrating when you get set back really far for it so I'd rather have some more checkpoints. Or else you can have more infrequent checkpoints but less difficult maneuvers, but each one feels like you could consistently do it so when you mess up it's just your fault. This blog from the creator of celeste is also a really good read on the topic of platformers too https://maddythorson.medium.com/celeste-forgiveness-31e4a40399f1
I hope this doesn't come off as too critical! That ended up way longer than I meant it to lol