Decay + healing does have some balancing issues.

I didn't see it on the worm itself, but for my fight with the Witch in the cave, somehow, she got decay on her (through mostly full armor of both types!), and so I was able to cast rejuvenate on her with multiple characters (which once again ignored armor). Might have been a bug, but I definitely didn't chew through her magic armor before murdering her with reverse healing.