Frost has had some bad crit scaling for a long time now due to how KM works.
Unholy Crit is so bad that pieces without crit, even a good item lvl lower, are still taken over a higher ended piece. Unholy has 0 reason to go for crit at all.
I think there should be new passives added for UH and Frost.
Frost = a portion of your crit chance is added to your mastery or crit damage.
Unholy = a portion of your crit chance is added to your mastery or crit damage. (This would at least help with the auto crit sudden doom procs)
DK needs a real rework in general, but at least those minor passives would help some until it happens.