Feats in GAC not properly counting

As Christoph says, it's not enough that they die. They have to be killed BY GUNGANS. Volatile Accelerator disks actually hurt you here, or at least they can. As part of a proper setup which allows you to rage through and kill someone before they get a turn, they're great. But if you can't quite kill them with your attacks before they take a turn, they're likely to die of DoTs and thus will not count for the feat. 

This doesn't mean you should avoid VA disks. They're great in most circumstances. But for feats that require you to kill enemies with a specific faction or character, you might wait to finish those feats at the end of Conquest once you've done everything else. That way you can remove your VA disk that is so useful for making progress and defeating bosses but is counter-productive here.