I would like to preface this review by saying that I'm purely talking about the impossible difficulty. I have not played the other difficulties.
Unfortunately, the charting on this one is mediocre. The first part of the chart is very uninspired; it is just a long train of on-beat bats. This part lacks the clever charting that made (most of) the originals so fun and satisfying to play.
The second part features nasty off-beat red harpies that are mapped on random notes played by a piano. I find this design choice to be weird for two reasons. First, it makes the chart unsatisfying to play. Some of the red harpies are placed on strange intervals like .26 or .38. Second, these notes are not even present in the OST, so why suddenly include it here? This seems like a poor attempt to make the chart stand out from the others by placing these red harpies on strange intervals, but it is not well executed.