Any one of the Forsaken is an incredibly lethal threat. That being said, I do think he's a bimbo, albeit a very dangerous one.

I think the problem is that not enough Sisters are as suspicious as Moiraine had become by the time Book III came around. She would have figured this out in short order, no matter how obscure the clue. It actually seemed like an Act of Plot Convenience that neither Egwene nor Siuan figured out the connection between Egwene's headaches and Halima's massages and the rumours that Halima was going around breaking arms and etc. Siuan especially is not one to lightly dismiss rumors like that, yet she didn't bother following up on it.

I just don't see Aran'gar being a match for Demandred, Graendal, Mesaana or any of the remaining players. I think there is a reason that of the little information we know about him, all we know for sure is that his intelligence network rivaled Moghediens', and I think that reason is because he is not as much of a threat as he is a tool.