The weakest castle in Heroes of Might and Magic IV!

Look.
Death can fully utilize its potential, for example, against Chaos and almost 100% against Nature.
Against Life, Death is effective by about 40%.
Here.
But, as I've repeated, this is said from the perspective of Life.
And so, it's optimal for Death to build its army in this way against Life.
Skeletons (effective), Imps (effective (because they disrupt Angels by draining mana)), Cerberi (moderately effective), Souls (effective (they constantly age soldiers, and you need to periodically remove the aging effect, but the hero can be killed)), Vampires are not effective, Bone Dragon (not effective (no one will focus fire on it; they will simply attack with Crusaders and Marauders)).
As you can see, Death's potential against Life is very low.

You can build a quality and effective attack based on Vampires, Bone Dragons, and Devils. Skeletons and Imps are mostly cannon fodder. It's not very effective to use them. Ghosts - yes, they can weaken enemy warriors. Cerberi - effective if you can quickly get them to a group of units. Vampires - not only do they fly and are invulnerable, but they also regenerate health by draining others! Moreover, a Grand Master of Necromancy is useful on large maps in large numbers. Devils - they can kill the enemy hero or at least remove their Immortality. They can also target archers, etc. Bone Dragon - hits living units without retaliation and also frightens them. It has very high protection against ranged attacks.
I don't know why you started considering the best Necropolis units as ineffective. But still, they have quite good chances against Life, at least.