I agree -- my personal opinions differ very widely from what's generally accepted as "premium", too, but unfortunately, that doesn't make it premium to everyone else. I certainly don't consider "Y" of low quality. I also think H and W should switch their classifications on the chart. No need for me to go into detail, though.
Z and Y and X to some extent to me are not anti-premium.
K, J and Q are terrible letters to me, everything else can look good. I've yet to see a good looking name with a J or a Q and very few with a K. Whereas all the other letters can be great in some combination's. Premium and anti-premium letters are silly, it all depends on the domain.
Sometimes I see Y as top quality. As in Gaby dot com .. but sometimes It's of less quality in Mygl dot com
The non premium letters a Q, X, Z, V, W, J.. it surely depends on where the letter ranks in the bigger word "Does it mean something..how it spell ..etc also add weight"