The big eyed art isn't actually a problem at all. That's a style question. I am rather fond of Itaru Hinoue style but she needs a lot work on her anatomy and I mean a lot. Chihaya in particular has some incredibly ugly sprites her arms look like wooden logs that has been pasted upon her body without actually being connected to her body. She also is very inconsistent with her art which lead to her characters often looking vastly different from shot to shot.
And why they aren't changing the artist. They have sometimes Planetarian isn't done by Hinoue, not Tomoyo After either. And Little Busters/Kud Wafter is half done between Hinoue and Na-Ga. But obviously they can't kick her. Why should they? She is just as much a part of Key as everyone else there she is not some contracted artist like Fumio for Tomoyo After. Kicking her out of her own freaking company would be awful. She is improving though it's going slowly. Eroge artist don''t tend to get better at the same speed as other artist because they don't practice much. If you look at the cg's from various eroge you'll see that its always the same sorts of angles used. Since the artists spend their time drawing characters looking exactly the same they simply never improve that much ):.
Well eroge/VN art in general is pretty fucking awful (Though colouring as gotten a lot better in recent years). And Hinoue isn't even near the worst of the lot.
This Review by Choux from calamitous intents details some problems with the art in Little Busters.
I could also mention the ugly outfits designs but those have actually improved. The casual clothes in Rewrite looks nice, stylish and actually fitting for their character (apart from Chihaya's yuck kill that thing with fire. Also Akane's clothes look kinda weird but nearly all clothes in VN's do so I guess that's expected)