^Hikki is far from being an amazing vocalist, some of her performances has more misses than hits vocal-wise... She has no control, breathing issues and is always composing songs out of her range... Still can put tons of emotion on her voice and shits on everyone else when it's about writing and producing music.
I don't care about lip-synching if the performance is good enough, like this one...
Ayu mostly lip-synchs during energetic performances, physical stunts and songs that are out of her healthy range nowadays, IMO they are all valid reasons for doing it, and even so she sometimes sings even with choreography (like 1Love during Future Classics, live vocals with heavy support from the back vocals). She is a performer more than anything else (even more than she is a lyricist IMO) and that's what I expect from her... Still, she sometimes take amazing advantage of her vocal limitations in order to bring rawness to some performances...