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

Well, that makes sense. At least zombies are indeed weaker than skeletons and vampires, although I would argue about ghosts. Ghosts are quite useful due to their abilities, and you can sometimes risk using them, but they are still not very strong. And what's the point of comparing units of the same faction among themselves, rather than with someone from outside? After all, troops always act together, and zombies work well with other units, being the most durable "meat" in the Necropolis, holding out until the very end in any battle. And regarding the argument about wyverns, it's artificial, because a Necromancer can easily achieve the number of zombies I mentioned, while it's not always possible to get a lot of wyverns, even if you really want to. I won't even mention that in some situations, pao can control the 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 when clearing neutral creatures... But every faction has such units, and they are not always slow.