The Cho-Ko-Nu's collateral damage works well in aggressive defence - that is, attacking stacks of units. With more survivability than a Catapult, and a strength bonus against Melee units, the Cho-Ko-Nu makes a great stack-destroyer. At this stage of the game, it makes the Chinese hard to attack.
Cho-Ko-Nu aren't so effective attacking cities, as they cannot be promoted in City Attack promotions. However, they can still be used as part of a combined-arms force, protecting attacking Melee and Siege units from Macemen and similar threats.
Defensively, the Cho-Ko-Nu loses its advantage. Alone, it's easily picked off by mounted units, and in cities, it's not much more dangerous than a Longbowman. If attacking China at this point in the game, give some Crossbowmen or Pikemen Cover promotions to protect stacks, and give stacks some Medic promotions to counteract any collateral damage.