The strongest castle in Heroes of Might and Magic IV.

But if the necromancer is a human (who has played the game before) and not the computer, and there are many neutral creatures on the map that can be used to increase the number of vampires, then it becomes very difficult to counter them. (In my opinion, the Necromancy amulet doesn't protect against vampire regeneration.)

And I'm telling you, Nature can triple the size of its army. Therefore, in this sense, it will be even more numerous (and, most importantly, more diverse).
As for the Necromancy amulet, it won't protect against vampire regeneration, but... it also won't allow summoning any more creatures (for example, demons to drain your hero's energy, or other undead). This is a powerful advantage.

That's true, vampirism doesn't affect them...
but building dragons means sacrificing Titans... then you'll only have hobbits and djinns as archers, and even djinns won't always shoot, as they might need to use illusion... and without archers, it's bad.

You have to sacrifice something to win against hordes (in the literal sense) of undead. Although, again, I allow for different options. Mechanisms will undoubtedly be the best way to counter Death. But then the enemy hero will be too far away. And a Titan could take him down in two turns...
Again, Order can also counter illusions. Vampires will also suffer from them.