Ok to just kind of some up these posts into one.
Like I've said before and on more than one occasion Cain has exhibited what could be clearly defined as moments of pain only to have him so no effect to the attack at all. Like I mentioned before Nightcrawler throwing a torch into his eyes and acting like it hurt even though he has had molten metal poured unto him with no effect or show of pain.
And yes those punched from WWH Hulk and Prof/Merged Hulk would fall into that category. You may say but clearly he said something that indicated it effected. While normally I would agree with most other characters with Cain you've got to take those reactions with a grain of salt considering his history of acting like things effect him when after wards they clearly haven't.
As for Nightmare, the Nightmare in that arc was when he was about to f up all of reality and had just beaten Dr. Strange and was mentally mind rapping him to keep Strange from freeing Eternity who Nightmare had supposedly captured. Cain was able to stalemate that version of Nightmare.
Like I've said Cain's attitude and mindset at the time tends to be the determining factor with how he interacts with attacks and other people.
As for Captain Universe I'm starting to notice a trend with things like these. I an not certify this for certain but I think character calling upon higher level mystic powers can effect Cain based simply on the fact their presence alone maybe enough to screw with his connection to Cyttorak.
Like War Hulk w/ the Celestial Technology or Shatterstar's sword or the Captain Universe. I'm not saying its a guaranteed thing either because I've always thought the more focused Cain is the stronger his connection becomes the more power he takes in. Like in the next issue Cain was able to go Toe to Toe with Captain Universe and was winning until CU had to resort to mind whamming him.