The problem isn't that they are fantasy China and Russia (it's just part of the problem), it's that their mechanics are bland. Take the Wu Xing Compass for example - why aren't the two siblings fighting over its use? Isn't it a unique thing? Lore-wise they are at odds even if they are working towards the same goal, yet mechanically it's not the case at all. Not to mention that they don't have their own mechanics. Paris and Hector in Troy fight over their father's approval, so they share that mechanic, but they also have other mechanics of their own. Not only is their "main" mechanic much more dynamic and interesting, they also get their own shit to play with. They channel this with John Constantine and Elsa, but as far as I can see it's very barebones and not like Paris and Hector's feud at all. On top of that, Boris shares this mechanic as well and it makes no sense with him.
The four demon factions have like, what, 1 mechanic each? Khorne is Taurox 2.0, Nurgle is Grom's recipes 0.5, Slaanesh seduces one (1) unit, only Tzeentch has true potential to cause chaos. I'm also questioning their inclusion/separation as well. Chaos Undivided could've had all these mechanics (and it does) and choose who to devote himself to to get better at that thing and unlock more god-specific mechanics. The factions are spread too thin with not enough ideas to go around, they are myopic and overly specialized, leaving no room for player input on how to develop the faction. Chaos Undivided is the only exception and it doesn't go far enough with this, making it possible to unlock every demon path muddying the waters again. When you choose which faction to play, you are de facto choosing a playstyle at best and a mechanic at worst.