The nerf is the fact that only (at least in co-op) two people can possibly use it at any time.
It's a beast, but it's not that hugely powerful. Just really good in the hands of those that know how to use them properly. Can't tell you how many times I've used shotguns in the game and managed to end up bottom of the scoreboard purely to forgetting to reload, missing shots and the huge pain in the butt it is to have to rack a new shot in that takes a bazillion seconds (at least it seems to when you're being swarmed).
Besides that, the only thing that makes it OP is flechette rounds that tear through armor and slugs that make shotguns a little more long range. I don't use slugs in co-op as they tend to be useless (or rather... not a tactical move) at short ranges.