The mirror classes are not exactly mirror.
If i understand correctly, the base of IBs "grudge" mechanics is damage taken.
Judging by the description, BGs "fury" mechanics may be based on damage done (to one target

).
Zealots have more offensive debuffs, while RPs have better defensive buffs.
Order needs more tanks, says Mythic and i agree. Our front lines are weak to non-existant. Having more tanking classes may improve the situation, provided not all KotBS are re-rolled SMs.