The game feels very limited by its own design, and the charts show that. The charting in general feels very samey and sloppy, if you've played a chart in Cytus 1 you've played them all, with no progression between chapters. Very often, a chart would have a very sudden difficulty spike for no real reason, or have very long intervals with literally 0 notes.
The fixed scrollspeed hurts a lot of charts very much, rendering some of them barely playable. In general, the game lacks a lot of settings, or any at all really.
The game also feels like it's missing a bunch of difficulty levels, there are a lot of charts just cramped into level 9. An entire new difficulty (like easy and hard) would be very welcome too, which is something I think is addressed in Cytus 2.
There are also some special story songs (the Alive songs), songs that are incredibly long (easily hitting the 5 or 6 minute mark on multiple occasions), but because you have to wait mid song for the story text to scroll, or look at a CG for a few seconds, then continue the song. The story itself is not that deep either, I don't understand why they chose the worst possible way to present it. Plus if you wanted to grind these songs it just adds unnecessary friction. I don't mind the length of the song, I just don't like the long waits.
Songlist is very hit or miss, mostly obscured by the poor, repetive charting. Even if the song is good, the charting often focuses on repeating one single pattern over and over.
I'm also not a fan of the whole pixel hunting mechanic for accessing the hidden songs, it's not properly conveyed and many of the inputs you have to perform are impossible to guess.

I played this quite a bit on an an iPad. Excellent song quality and variety, and beautiful song artwork.

This game used to have song packs unlocked one chapter at a time every 100,000 downloads, until the fulfillment of the 1,000,000 downlaod campaign, which was met. Now only a few song packs are locked by DLC and over 180 songs are available with the initial purchase.

I like the scanline note presentation and it's one of the few rhythm games where I don't mind the speed of notes being based on BPM. The timing windows are fairly loose, which might be a turn-off for more "hardcore" players but there are certainly players who don't mind it too much. Also, the difficulty scale is out of 9 and there's a huge spread of difficulty in the level 9 range. Also scoring is combo-based, with all the problems that entail (miss 1 note in the middle and it's worse than missing several notes at either end); there is an alternate scoring system called TP that is accuracy-based, but it's heavily de-emphasized in favor of score.

Frankly I prefer this over Cytus II. Cytus II is too grindy and reliant on DLC purchases.