Balance in HotA.

Nothing, the point is precisely to create competition for the wings and avoid situations where one player has them and the other doesn't.

By the way, if templates with water are ever implemented, portals to troops would fit perfectly there, because there are no roads on the water, the geography is always inconvenient, and it's inconvenient to build chains between land and water due to the penalty, so no one might go into the water at all, and with these objects, there would be at least some motivation.