I don't have enough experience with Bannerlord yet to say for sure, but it does seem like geography is a big issue for Sturgia. The campaign I'm currently playing is only on day 70-something, but Sturgia has already started to lose pretty badly, with Tyal, Sibir, and some castles falling to the Khuzait in the east.
The Sturgians were dominating almost all of the open field battles against the Vlandians in the starting war, and traded Nevyansk Castle and Caleus Castle back and forth a couple of times. I actually signed on with the Vlandians as a mercenary, but my army was much too small and weak to even think about turning the tide - the Sturgians were fielding some pretty high tier units, and in significant numbers. But once the Khuzait and Northern Empire both declared war, it all started going downhill for the Sturgians. They eventually made peace with Vlandia, returning to the initial borders, but they'd already lost numerous parties to both the Khuzait and Northern Empire (along with some of the aforementioned settlements).