First of all, it's great that people are finally making Rhythm Heaven inspired games. It's a subgenre of rhythm game that, as far as I know, has had no other entries besides the RH series.
This game does a commendable job, it doesn't reach the highs of the RH games for various reasons I'll talk about below, but at its peak it does have some moments of matching them, which makes it a worthwhile play.
The game's aesthetic is really unique (I think it's called lo-fi...?) and they do a great job with it. Combined with the music style, it really gives this game its own vibe and you come out of it feeling you had a unique experience.
The downside is, because the music fits this lo-fi sleepy aesthetic, it... kind of puts you to sleep. Some of the games have vibrant, exciting music. Others are drowsy. The music is never bad. It's just great music for relaxing and not for maintaining perfect focus on a precise rhythm level.
I thought World 1 was by far the best world. When I played this game's demo, I had no idea what it was, I just checked it out 'cause it said Rhythm. When I realized I was playing a Rhythm Heaven clone, I was blown away, in pure delight. Such a pleasant surprise. 1-1 is a fantastic intro level that really captures Rhythm Heaven gameplay and complexity. 1-2 is also really good and remains one of my favourites. 1-3 is pretty good. 1-4 I didn't love. And then Remix 1 was... awkward.
When the full game came out, I was really excited, and it... kind of never recaptured the feeling of World 1. Which is very sad. But a lot of the later games are just... too simple? And they don't really do anything complex. Most Rhythm Heaven games take their base concept and do unique things with them, switch up the timings a bit, escalate in intensity, etc. Melatonin games mostly don't really do that, it's just the same basic pattern over and over. Combine with the sleepy music and it can be... a bit dull sometimes. (I'm not sure what the idea was, for example, with the Claw Machine game in world 4- it's a very simple concept, but then the climactic moment is... just doing nothing for 10 seconds??? I think it's supposed to, like, lull you into a sense of calm, and then when it's time for the next input 10 seconds later you maybe have zoned out and miss out? But it's really... not that hard...)
Another thing is that this game is actually more similar to the very first GBA Rhythm Heaven, in that a lot of games focus on specific button presses. Later games basically just had one thing to press and the challenge was keeping the rhythm, but in this game there are many games (basically all of world 2? and then some others) where the rhythm is very easy to keep and the main challenge comes from correctly pressing Left or Right based on the visual cue. This isn't bad but it's just not the gameplay type I prefer, and the game relies on it to the point where all those games feel pretty similar.
And the remixes stay awkward. They just don't flow very well. Maybe because the music is mostly all chill, it doesn't really have good momentum taking you from one game to the other. Transitions between mini-games are halting. There's lots of strange gaps where they have speed-ups or slow-downs when you've barely even playing at the previous speed. It's a shame because the remixes are some of the best parts of Rhythm Heaven. The only one I liked here was the Final Remix, it really did a good job putting everything together and makes the game feel like it was building to something, it wraps everything up in a nice bow and makes you feel you went through some kind of emotional experience.
As a note, my friend who is a rhythm expert said that the window for hitting a Perfect in this game is much tighter than Rhythm Heaven. I got all Perfects on all stages and it took a looong time. I didn't necessarily like the game enough to wanna go all-in like that but it's still a Rhythm Heaven game and I don't count them as complete until I've got all the perfects, so...
It's still worth playing for sure if you like Rhythm Heaven, it basically executes the same gameplay and even if it's not always thrilling, it's interesting to see this take on the genre and won't take too long to play anyway. It has at least a few parts that will stick with me.