One serious, serious balance issue with an area attack on your own cards with Aflatoxin... Vultures.
Take this scenario...
You have a field full of malignant cells, plus an immortalized vulture, with boneyard and empathic bond in play too. Boost the defenses of one cell so it can survive, then blast your field with two thunderstorms in the same turn. As much as 42 attack added to the vulture immediately, plus you have new cells popping up for a second round of that in a few turns, assuming your enemy didn't die.
84/85 creature, anyone?