I finally got around to test this wonderful mod. I've no experience of Avernum though, so I take my comments with a pinch of salt.
1. The Vahnatai warrior has a suspiciously shorter life expectancy compared to the other Vahnatai units. Is this intended?
2. All your casters got higher attack, strength and defense then mages in other nations. I assume this is because of the setting?
3. The Avernite Mage is a bit of an oddity compared to the other casters. I found little use for her. The Nephil Shaman is a much better researcher (holy ability makes him much cheaper in upkeep). And all the other mages are more competent in battle. I'm not sure how to make this unit more attractive without breaking the theme. But you could perhaps give her access to all paths of magic, as per the units description, or make her the "General" unit with a higher command ability (normal or magic units).
4. Why do the Slith units and the Vahnatai Soulguard have mapmove 1? I assume this is because the Slith are sluggish lizards (though that isn't explained) but why is the unarmored Soulguard such a slowpoke? I reckon it's because their reluctance to leave the ancestors unguarded or because they are not trained to make long marches.
5. Avernum is a nation that lives under the ground, but can't see in the dark. In my game I started out next to Agartha and promtly lost my capital due to my own forces fighting blind in their own home.
(I still beat the snot out of Agartha though, but that's not the point)
6. The Avernite Guard could use a bit little more oomph to set him apart from independent infantery. He looks like he's using a wall shield instead of a kite shield, and his helmet looks more like a half helmet instead of an iron cap. Then again if these guys become better protected you need to cut down on the PD. Makes sense to me though. These guys wouldn't have the numbers to field large defensive armies outside their home province.
7. Your choice of sacred units is nothing short of pure evil. These guys practically shout "Water 9 blessing, dammit!" but the Soulguard works so darn well with Gift of Health that it brings tears to my eyes. So of course you made the nature blessing completely redundant. Kudos to you for a great balancing move.
That's all for now. Expect more feedback as I make my way through early game. The units look so good that congratulations for an excellent crossgame transfer is in order.