Native improvements vary by map (excluding the ones obtained via the Federal cards Mexico has, but those specifically help others units so I’ll ignore those for the moment).
HC shipments aren’t “situational” in treaty mode. You can always have the option for Fully Upgraded Chinacos (and realistically Salts and natives) in a single deck.
Chinacos don’t straight up beat FU Musketeers, what they do is cut what should be a massive win for the Musketeers to a modest one.
“But Chinacos are lancer cavalry! They’re supposed to trade better into Musketeers!” You say.
Lancers are supposed to lose harder to Hussars, and Chinacos don’t if you conduct an actual test with full upgrades (FU, or “Full Upgraded” meaning all techs and cards available that buff attack, and or hp within the homecity, or buildings available to you on any map) and an upgraded flag aura. In fact, at that level, Chinacos will have a significant advantage (and “significant” here, could mean a margin as small as say, 10-20% of total hp left after an even pop fight.). It’s also worth noting that Dragoons are less-used by most civs in treaty as they’re quite pop inefficient. If you say “just make dragoons”, half the civs won’t be able to trade efficiently into a mixed army comp (especially when Mexico might have like 50% more units already).
A unit that kills Skirms faster, takes a much-less-bad trade vs Musketeers, and soft counters other heavy cavalry is concerning in its own right. But as I’ve said numerous times now, that’s not the only overtuned thing Mexico has in the late game. Their skirms, their pop efficient eco, their guaranteed 35-40 pop equivalent of natives, all work together to make a civ that’s unmatched in treaty.
On their own, Chinacos would probably be fine, but when you look at all of it together, it’s stronger than any other civ in treaty.
Now, please, look through the below and explain to me how all the below changes, taken together, would unreasonable hurt the civ in a 1v1.