The O&G go-cart list, which consists of tons and tons of tiny drops that are either incredibly annoying (pump wagons, chariots, spider riders etc.), or strangly effective (mangler squigs, hoppers, fanatics, small units of trolls), can be really hard to play against. Throw in a couple of bad moon templates bouncing around the table and you have a ton of things to try and deal with at once. Animosity still really hurts them, but it's not enough to cripple the army.
I wouldn't say they are overpowered, but they have lots of neat new tools which can be a pain to deal with. Keep in mind that their army can do as much damage to itself as it can to you, and don't get frustrated when you lose to a luck-based army.