In my experience, this is the list of Nations from strongest to weakest:
1. Khuzait
At first, they seemed underwhelming until people started the Khans Guards and cavalry spam. Any sort of enemy infantry will die once they get surrounded by archers. This is especially easy on the desert map. Enemy cavalry will loose some horses in the hailstorm of arrows and the one that get to the Khans Guard will be chopped to pieces by the knock-off Naginata. To quote a player from a game of captain-mode I played: "Khuzait feel just like cheating."
2. Sturgians
The infantry is a bit weird. You get the 16 Varyags. Why should you pick the brigands where you get the same amount of troops? The same goes for cavalry, both weak and strong cav get the same amount of soldiers. Why? Why would anyone pick the weaker one? I think the number of soldiers for Sturgia must be adjusted, I don't think the reason for them being so strong are the troops itself.
3. Empire
They got both, good cavalry, good heavy infantry. I think they are (together with Vlandia), the most well rounded factions, which makes them appear pretty good, but I think they are the middle-ground that should be aimed for in balancing the rest.
4. Vlandia
I think this is the middle-ground the other factions should be adjusted to. I don't think anything is way too weak or way too strong.
5.Battania
Weak infantry, the cavalry is a joke. Fiaans are the best they got, but against any shield infantry not so good since flanking with them is quite a challenge because they are infantry. Oathsworns are okay but will loose to Legionaries and the like. The Savages are the biggest joke out of their trooptree. They lose against Berserkers all the time. Realy, really hard. They got 3 amour (improved 12), while Berserkers have 15 which goes up to 24 with improved amour.
6.Aserai
The only faction I see Battania regularly win against. Their best melee infantry does not have shields. They get slaughtered by archers. All units they have are okay, but nothing more.