IIRC

Amazons can get them from their weapons(may only be certain types).

Assassins can get them only on certain types of claws, I think its exceptional and elites

Paladins can also can get them on their shields(may only be certain types)

Is it unbalanced?

Necros aren't exactly unbalancing and besides they have quite a few skill types that have to be used together to do good

Sorcs need versatility since immunes and besides they cant use a orb and a staff at the same time

The paladins don't really have that many skills they use anyway, attack skill aura and holy shield, not too unbalancing

Besides since when do variants ever worry about being balanced.

Correct me if I'm wrong