Necropolis or Castle (Which one is stronger?)

Take a human army, add a couple of skeletons, some kind of wyvern, and see... Failure after failure... Meaning it happens! But for necros — never... Which is exactly what we're talking about.

This is all correct, of course :)

But, as a rule, when it comes to the final battle, one hero has all undead, and the other has all humans.
I don't exclude the presence of one "random" stack of very powerful units in the army, but by the final battle, any competent hero will either pick up a couple of artifacts that increase morale, or visit a few morale caches, or have Leadership, etc., or maybe all three :)
But for a necro, no matter how you spin it, forget about it! The only way out for him is an artifact that removes the opponent's morale bonus.

So, it's more likely that the humans will be inspired! :)