i think still my #1 complaint about osu!mania is having to play 8K with ONLY TWO COLORS FOR NOTES
although nobody will see it as a serious complaint
also i disliked the scoring system
hold notes in general (again, not a serious complaint)
peoples' charting styles are bad (not a serious complaint)
nothing good was ranked (might be different now)
that's all i got for now
in all of those things i listed above, i liked FtB's system better
also i liked FtB's scoring system (basically before classes were a thing) because the 1x-5x combo-based and 8x made it different
from "whoever gets the best accuracy wins"
which is every other game ever
no complaints on FtB3 classes other than score increase
but yeah
if i had to rank the above reasons of why i didn't like mania in terms of why i hated it
1 - only two colors, hard to play
2 - community was terrible
3 - charting style, didn't like any maps
4 - hold notes
5 - scoring system
6 - not enough ranked maps