It's not so much issues with Björk's character so far as I can tell, it's more just that they think it means Yuki isn't being original and different and provocative. Personally I don't think that comparing them means either of them is lesser.

My opinions on Björk's music vary considerably with different songs and different albums. Some I quite enjoy, some I'm ambivalent about, and some I don't like. I've only seen a small handful of her videos though. From what I've seen and heard so far by Yuki, she's more consistently to my taste.