[Taqwus]

Ulm is fairly straightforward. Arco or Pythium combine strong mundane power with versatile magic as well, but it takes some time to learn the magic system -- e.g. to appreciate the power of communion.
Ermor, in either Ashen Empire or Soul Gate themes, is very very different. Bad habits there do not necessarily work with other nations. Countering either of these themes also tends to differ from fighting other nations.
The underwater nations are a bit 'safer', at least sans Ermor and the other underwater nation, as it takes landlubbers some doing to get significant amphibious forces, but R'lyeh at least is a bit strange to work with.