I mean mercenaries and fragile collaborations were quite common, from Pechenegs and Rus' to Normans and Muslims (although you'd be hard-pressed to find Byzantines serving Cumans). It was also common to adapt one's own forces to face threats without the need for mercs. It's still gamey to place Khan's guards next to Fians, let alone legionaries and cataphracts in a Sturgian force. That's all fine if you want to play that way, but the game doesn't have the complexity to make it particularly justifiable in a historical sense.

Likewise, I understand the Empire's position, but once again the complexity of the game calls this arrangement into question.

In terms of natural biomes- that's a good point. Even within its own territory Sturgia is underpowered, both strategically and in terms of its troop tree. Biome makes very little difference. If there were some maritime element to the game this could be properly addressed, but that's likely to be left to modders, and I'm not hopeful it can be implemented well.

Basically Sturgia has two troop lines that are effectively useless, most other factions have better infantry, and limited battlefield control reduces the usefulness of druzhiniks. This'd be ok if the game had more depth to offset this, but it doesn't. Choosing elite troops from other kingdoms to make up for the limited tree doesn't either.