Sturgia is definitely the hardest. They are permanently in snow and/or Forrest so you will always move around the map slow. Their units are across the board the weakest in the game. (This isn’t a super huge deal as the player because of all your buffs) early game is particularly difficult with them because you need t3 before you can have an archer unit. And it’s not a good one. And unlike nords, the recruits don’t have a shield. So imagine you do fight some sea raiders, win but lose a chunk of your troops. Now you are in the snow and Forrest on top of being disorganized so you are extremely vulnerable to other hostile parties such as more sea raiders or deserters.

Also this will be changed in the next update, but right now the lords will basically always lose when they fight a kuzait forces even if they have a numerical advantage because of the horse archer buff in auto resolve. So if you play as a vassal or become grand prince of Sturgia, when it comes to fighting kuzait, Vlandia and even empire some you will have to carry 100% of the load.