Don't like it, looks like a blatant Borderlands 2 mechanic rip off but oh well...

You mean when Legendary enemies mutate? I find this mechanic interesting but not really necessary. I just knock them down a couple of hits, when they mutate, lob Molotovs or grenades and they're toast.

Plus, if its a previous Fallout mechanic, technically Bethesda, since owning the rights to the Fallout franchise, can place it in the game. Plus it adds depth to characters.