Honestly, there are some of your icons that I absolutely adore. Your last sample is definitely one of them. The colors are gorgeous, it's not too busy or too plain, it's just fjkfs;.

--But then you've got things like your third one that kind of just... ugh. The coloring's fantastic, and the colors you use are still gorgeous, I just think your icons look a little flat with just that alone, you know? Maybe try adding some gradient soft-light layers, or a selective color layer to slightly change the color of the lineart, so it doesn't stand out against your color so much (or you could change it so it stands out MORE, but that's up to you). Just sayin', black lineart is so zetta boring. you know? I want to see you experiment!