[Agrajag]

What you are saying is that the advantage sea nations receive in their exclusivity of provinces is balanced by their hardships in taking land provinces, while cave nations have no disadvantage to balance their exclusivity on cave provinces, except for Agartha.
Well, I say that since cave provinces are so few, the advantage isn't that big. Its only considerable where there are "landmasses" of caves, and those would tend to be for thematic purposes (like underground passages), in which case balance isn't a consideration. Furthermore, a disadvantage in "light" lands would make these nations much weaker, since they still rely mostly on land provinces. (again, except for thematic maps.)
This would make underground units more of a perk (like free scrying) than anything to really base a strategy around.
The last argument I have for now is that darkvision is only useful under darkness, which currently is a D4 spell costing 4 death gems, which makes it a rarely used skill. And even Agartha which is a darkvision race, has a hard time casting darkness (one in 2 400 gold mages requires two boosting items to be able to cast it.)

Finally, which nations do you consider as cave nations?