for one.. penitence or whatever its called is a ranged skill with more then 1 function...
Its a interupt not a stun, it does damage, it slows the other player and i believe it also removes some armor. quite a nice list.
While erebus' skill is an actuall stun. it stuns everybody, making them unable to take any action for what ? 3 seconds at max rank or something... Its only at lvl 15 that it gets a cooldown increaser on top of it
So sure, his skill is AOE, but all it does is stun, while oaks skill is a whole bunch of things in one, and not a stun but an interupt.\\
Sure.. this doesnt automaticly mean it is working as intended... I dont even know if erebus stun works while shielded personally.. but this hsould be some food for thought