I feel like immortal is like viltrumite foot soldier level, even his powers indicate so. Immortal might literally be very similar to viltrumites in his genetic make up or whatever after he got blasted with that orb thing thousands of years ago (it was in a shot of I think season 1 , montage of his life, when the mailers Brought him back). But yeah OU (our universe) mark has something other marks don’t seem to have. Something…evil? Dark? A certain rage? Remember in season 1 there were like 1 or 2 moments during fights for example with the Flaxxans where he snapped and went super Sayan. Now this season he released some of his darkness with Darkwings sidek-apprentice. It might truly be a mental thing.