Frequency has some gameplay issues. Notably the fact that if you do a one bar track at the beginning of a 8-bar section, then do 3 2 bar sections, you lose your combo. I really hate that. I also dislike how at the end of a section, you're never sure if the track you're completing is going to complete the same track in the next section.
I do like frequency's songs and patterns better, though. Amplitude has too many songs with lots of repeated notes, which I heavily dislike.