If Clark's molecular structure is much denser than a normal human's is, wouldn't that make him MUCH heavier than a normal human his size? And that could account for alot of problems. Another though: in Ultrawoman, where besides superman two other people had his powers briefly (Lucille and Lois), did they have his molecular structure sort of transferred to them to make them invulnerable? Some of this stuff just doesn't totally make sense.

Mulder: Imagine if you could come back and take out five people who had caused you to suffer. Who would they be?
Scully: I only get five?
Mulder: I remembered your birthday this year, didn't I, Scully?

(The X-Files)