"Necropolis" - changes to creatures of levels 6-7.

A common misconception, not just among the creators of HoMM. It is believed that "so-called" skeletons are made of decaying bones that crumble from even a light blow. I completely disagree with this.
A true necromancer, wanting to create a truly powerful unit, will reinforce its bones with dark energy to such an extent that it will be impossible to break them at all. Have you seen Ghost Rider? It's essentially related to this indirectly.
And what part is actually missing? Only the skin, muscles, and internal organs are gone. Of all this, only the muscles, ligaments, tendons, and the like strengthen the bones themselves; all of this is more than compensated for by good dark enchantment.
As for the heroes, I'm not arguing that the amount of HP doesn't matter here. But all these units are just a quick fix, a couple of mana points spent on a crowd of weak skeletons, when it's not a problem to spend a couple of thousand and create one - but stronger than an entire army.