Hello,
I will give my opinion on this subject, I am a 100% Sturgian player. I never take a unit from any other faction, except the Tavern Mercenary. I always play on realistic difficulty and it is stupid to reduce the difficulty, it is a false solution.
For the Sturgiens, they are not as efficient statically as their neighbor. But the real problem is always the same, the AI does not know how to play with sturgien who are dependent on their formation. The Sturgans lose troops en masse as soon as the AI orders to charge.
I love playing sturgia because it's hard ... because of losing a battle 1000 sturgians against 800 batanians or empire, it's really painful. Against the Khuzait, 1000 sturgians vs 600 khuzait, the sturgians are tied or lose.
To win you have to use the land as much as possible and do plenty of micromanagement.
After the Sturgians are a mixture of the Nords and vaegirs and the vaegirs had the best warband archers.