the main problem is that the covenat have less ways of taking it out than the UNSC.  So when the arby is playing a covenant, it is OP because you have to do one tactic which gets owned if he is going wraiths. 

I think he is OP because he was meant o take out armies, which makes him a very good leader - better than the prophet - and that his unique units are not the worst. They should be but this would not balance it out.  His rage is just too effective for n0obs to use, and thus this makes it OP