Yeah, this for me also.
I mean, when you look at the weapons in Halo 4, more specifically the BR and DMR, the Needlers would have to be powerful. Otherwise there would be a lack of variety regarding what power weapons you can use in Halo, of course if only the DMR and BR, were the best, everybody would use them. But, luckily there's good balance with most of the weapons in the game. Well, Primary weapons mostly. I don't think it's OP, but it is bloody powerful at short to medium-ish range.
The needler is definitely a power weapon, the ammo disappears quickly and really you'll only be using it for a short amount of time. I'm not so sure how many kills you can pull off with it but I'd say at least 3, maybe 4. It could be more I don't know but to my knowledge, the needler doesn't have a lot of ammo. That's the main reason for my why the needler isn't OP.