charlie, check out this video i posted in another thread. this is the only way ive been able to instantly teach proper squat form.
http://video.google.com/videoplay?do...81301858251744
i'm inflexible as [censored] and can still squat ATF.
as for your question, hack squats are rough on my knees personally, and i can do every other variation without trouble. hopefully someone else can chime in.