I was immediately wowed by how stylish and gorgeous the art is! But, I had a really tough time playing this... (and this is coming from a rhythm game aficionado!)
* The menu navigation keys felt really awkward and unintuitive. I would have preferred if the game stuck to menu keymaps that people already had muscle memory for (arrow keys, start/space/enter, etc.) for example, 'F' feels really awkward for menu confirmations. Same with splitting up WASD for song choices (WS/AD for song/difficulty just didn't gel). I had to actively concentrate to navigate the menus, when it could have been intuitive/natural for players if the game tapped into the keymaps people already know.
* Also, including controller button hints (triggers, "(x)") was a little distracting if you're not using a controller. Having auto-detect could help to simplify the UI...
* My gut reaction was that were too many button possibilities. While great for flexibility, it made it hard to settle on a play style. I would much rather be able to choose between a couple fixed/limited options rather than having EVERYTHING (WASD, IJKL) available at once.
* Having to sit through the same cutscene every time you start up a new song was a little annoying. Especially if you're simply trying to switch difficulties.
* The tutorial has these constant interruptions every time you make a mistake. The interruptions were breaking the immersion/flow for me, so I skipped the tutorial and learned by jumping straight into a Normal song.
* The dodge "notes" felt a little janky/imprecise, because all they give you is a "nice" compared to the feedback for actual "notes". It felt funky to switch between trying to hit something and trying to dodge something.
* The note feedback is possibly a bit too granular. "Miss/Barely/Okay/Good/Great/Perfect+offset" is a lot of different states. I'd be happy with Miss/Good/Perfect (similar to what Taiko does with Bad/OK/Great).
* For faster/dense sections that include both L and R notes, I felt like there wasn't enough horizontal width to react quickly enough. The center stage takes up a lot of space, leaving not very much runway for incoming notes..
* The art style is amazing, but it felt imprecise for a rhythm game. To me, there was too much squishiness/fluidity for me to be able to develop the hand-eye coordination needed to lock into a groove. It was hard to tell how the notes aligned with the timing grid, even with the square of 4 circles placed around Beat.
* There was also just too much information/stimuli presented at once for me. Visual effects, camera movements, L-R perspective shifts, score, combo, percentage, time offsets, heartbreak meter, "Up Next" popup, etc. etc. It's gorgeous to watch, but as a player, it's really hard to pick out key information. When everything is trying to stand out, nothing in particular stands out, which is bad if you're trying to gauge how you're doing on the fly. (Then again, I'm the kind of person who prefers a really no-nonsense skill-focused rhythm game, so I might not be the target audience here. I didn't like Friday Night Funkin' either...)
I'm not a UI expert or anything (obvs)... all I've really done is casually watch a few Tantacrul videos...