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

Ah, yes, slow zombies with dragon stats and fast dragons with zombie stats. Everything is correct. And the level of creatures doesn't matter at all in Heroes.

Yes, level doesn't matter in Heroes.
Lower-level creatures are built earlier or (in the case of levels 1-2) additionally brought by heroes, and often by dwellings, so in the end, on a normal game timeline, a level 1 creature can easily outperform a level 7 creature in strength. How can you compare units of different levels based on their base growth rate if the number of growth rates for lower-level units in an army will be much greater.

In general, levels don't matter here; it's simply about comparing "all other things being equal," i.e., with equal Power for the unit. The catch is that the developers only take into account strength stats + initiative + some abilities in this parameter, but speed and most other abilities are not taken into account, so formally, a slow stack of zombies is as good as fast dancers, for example. But in reality, the advantage in speed is crucial, which is why all slow units suffer (except for those with strong uncounted abilities, like dwarven warriors, for example).

Added 9 minutes ago

Necropolis units, with a few exceptions, are weaker than their counterparts in other cities, which is more than compensated for by the necromantic abilities to create a huge number of troops. I don't think zombies are weak in any way: when the enemy has a little more than a hundred of the same furies or knights, and you have a thousand zombies, they will definitely compete for the top spot ;)

Uh-huh, with the following exceptions:
-skeletons
-ghosts
-vampires (alt)
-liches
-and in this mod, also levels 6-7
I.e., almost the entire army.

Added 9 minutes ago

I don't think zombies are weak in any way: when the enemy has a little more than a hundred of the same furies or knights, and you have a thousand zombies, they will definitely compete for the top spot ;)

You can say that about every unit. Wyverns are weak? Okay, okay, let me capture and build a couple of Strongholds, as well as a couple of external dwellings for wyverns, and my wyverns will "tear through" anything!

As for practice, raising zombies with necromancy makes sense only when the hero is Orson; under him, zombies play the way they should play without him. In all other cases, vampires, skeletons, and liches will be raised because they deliver on their Power, but zombies do not.

Added 18 minutes ago

Right now, that's the case. At least at the same level. The only exception is Furies, but there the growth rate in a long game is too small, so it evens out a little later.

As I already explained, the total strength of the growth rate in Heroes 5 is balanced by Power, which does not include speed. I.e., slow units are generally outsiders, and only strong abilities can compensate for this (which zombies don't have).