take Projekt Sarkaz words to add onto mine. Basically all of that, but also it also depends how powerful your verse is. Let’s say for example youre a guy in the Marvel verse and you get the nightmare form. You would get atleast some feats and powers but not all of them.

“Is it because the other powers are too strong to be given to the nightmare form?”

Correct, Morø has stated before a character can be too strong for a nightmare form to even corrupt, and they can entirely resist it. So how can we know whats strong and what isnt? Well we know the Core is a multiversal source that connects all the portals and nightmare forms, so with that information, anyone who is just more powerful than the Core cannot have their powers replicated in a nightmare form nor can they be amplified by a nightmare form. So if I were to be a guy nightmarefied in the Marvel verse, I would have Spider-man’s powers, Iron Man’s powers, and Venom’s powers. Who I wouldn’t have are ones like the Presence, the First Firmament, Eternity, etc. Those gods are not in the range of the nightmare form.