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

And the argument about wyverns is also artificial, because it's not difficult for a necromancer to achieve the number of zombies I mentioned, while it's not always possible to get a large number of wyverns, even if you really want to.

As I already wrote, the necromancer won't raise zombies (it's simply not profitable), except in rare cases (Orson).
Having a unit just survive until the end of the battle is not the best role for slow units.

For example, in my mod, slow units represent their main combat force and threat, while fast flying units have more opportunities, but physically they are much weaker. It should be something like that; otherwise (if a slow unit has the same physical stats as a fast flying unit), the slow unit simply becomes a second-rate cannon fodder. By "flying units" I mean units with top speeds; in previous installments of the series, they all had the class ability "Flight." In Heroes 5, for some reason, even infantry can have top speeds, falling into the "flying unit" category even without that ability.

I won't even mention that pao-kai can sometimes dominate an entire battle, and legends are told about this on this forum.
The only serious drawback that zombies and wyverns have in common is their limited usefulness in clearing neutral creatures... But every castle has such units, and they are not always slow.

Wyverns are useful in clearing neutral creatures precisely because of their regeneration. In the final stages, they are of little use, which is indirectly indicated by the format of the RTA, where they are rarely bought.
Wyverns are weak because they lack the "Blood" trait, and the developers, when calculating their Power, accidentally or intentionally confused them with executioners in terms of large size (i.e., executioners are calculated as large, while wyverns are calculated as small).