Sadly just stats are not the only issue I have with some units, certain passives make some of these units a problem even when compared with other Cathayan units. Of course my advice doesn't need to be taken fully into account but I do think it'd be good for it in the long run.

Celestial Dragon Guard (Blade & Shield): The only real problem with these is the passives, and the fact the stats are a bit off when compared with the vanilla Celestial Dragon Guard unit, I think the passives for encourage, immune to psychology, and defensive stance should be removed as the vanilla Celestial Dragon Guard unit does not have these passives. Likewise I think speed should be reduced to 28, charge bonus to 15, and armor to 95, to match the vanilla unit. The melee attack and defense stats are fine however. Their campaign upkeep should be 275 to match the vanilla unit, and recruit cost for vanilla and multiplayer should be reduced to 1100, they should also take 2 turns to recruit same as the vanilla unit and their hp should also be changed to that of the vanilla unit (though it's only 400 lower than the vanilla unit is on ultra).
Celestial Dragon Guard (Halberd): These ones are weird because we have the vanilla halberd unit already, as such these should maybe be considered Elites instead and be altered accordingly. Drop the entity count to 80 instead of 90, but up the hp such that they have 8000 hp on ultra. Up the melee attack to 40, and melee defense to 50, ap weapon damage to 25, give them 80 leadership, move them to the tier 5 building instead of the tier 4, and make them take 2 turns to recruit. No passives need changing as this unit has now been considered a more elite unit than the vanilla halberd unit.
Imperial Guards "Jinwu": Reduce melee defense to 60, melee attack to 45, armor to 120, charge bonus to 30, and reduce weapon damage, to 20 base, 25 ap, 10 bonus vs infantry, 20 bonus vs large. Reduce upkeep to 750, and increase factionwide cap to 10 (noticed they had a cap in the pack file), make them only recruitable from landmark buildings in the major Cathay cities like Wei Jin or Nan Gau.
Sea Dragon Samurai: Reduce upkeep to 200, remove stalk and defensive stance (cause honestly those contradict each other, they're stats are more offensive, so you'd be charging with them, as such defensive stance is almost pointless), give them fear instead, they're shock infantry, would be fitting for them.
Inquisition Secrecy Guards "Earth": Reduce melee defense to 45, charge bonus to 40. I'd say to remove terror but it's not really an issue if it stays.
Terracota Sentinel "Titan": Reduce armor to 100, melee attack to 50, and melee defense to 50.
Celestial Dragon Knights "Chosen": Reduce melee attack to 40, melee defense to 45, bonus vs infantry to 8.
Longma Riders "Yulin": Reduce melee attack to 45, melee defense to 50. Reduce the upkeep to 600, increase entities to 10, reduce mass to 1600, increase factionwide cap to 6.
Immortal Warrior: Reduce melee attack to 75, melee defense to 60, reduce upkeep to 1500.
Cathay Archers "Crane Gunner": Reduce entities to 70, increase ammo to 18, reduce upkeep to 300.

And those are the changes I am having currently in mind. Of course this will depend on if the creators feel this is reasonable. I've tried to not nerf Cathay very much in this, as in vanilla they're generally in a good spot for how their armies perform in campaign. But with great threats they may need better units than what they have. That said they can't completely stomp a faction like Khorne in melee because all Khorne has going for him is melee. Due to that it's unreasonable for Cathay to be as good as Khorne is in melee. So these alterations will nerf the upper potential of Cathay, but not so much it'd hurt them very much at all. They will also be able to afford some of these units more easily with their slightly lower stats at the reduced upkeep points, so it's a more net neutral change to Cathay in terms of campaign at least in my opinion. Let me know how much of this seems reasonable. I do really like the mod, it's just a bit hard to deal with some of these units when I'm playing Khorne or any other faction that doesn't have better unit mods out yet.

The Steam Tank Equivalent for Cathay seems to be better than the Empire's. In a Direct 7 v 7 the Cathayan tank destroyed all Empire steam tanks while only losing 3. The reason for this is that their passives are far stronger and more than the Empire steam tank.