AI have some deep problems with hot it behaves, but that's not exclusive to Sturgia. AI is the same for all cultures and lords. Btw, devs said that they fixed starving in 1.3.0 so I am surprised to hear that.
As for mounted troops, it's just 30% bonus and AI armies doesn't recruit too many cavalry anyway. Vlandian army made of 20% cavalry against Sturgian army made of 5% cavalry will have advantage of just 4.5%. That's negligible difference.
As for geography, parts of Northern Empire and parts of Battania have actually harder terrain and while geographic spread is true, it's not exclusive to Sturgia. Aserai and Khuzaits are spread out as well. On the other hand these realms have one of their border completely shielded, so while central realms re more geographically compact, they also face enemies on all sides. So there a pros and cons. You can really say that it's only Vlandia that have huge advantage over everybody else in the campaign.
All main line cavalry of all cultures starts at T4, with the exception of Khuzaits and noble lines are not common enough to make an impact in the oweral numbers. Again, you can say that only Kuzhaits have advantage here. But even with 50% cavalry, you are still talking about autocalc advantage of just 10-15%.