Combining the versatility of the dagger-axe with the long reach of the spear, the ji is a superior weapon in many aspects, functioning as a pike, war pick and bill hook combined in one, making Ji Infantry a powerful unit, especially with regards to fighting cavalry, especially chariotry. They are not ideal for siege work, but vast numbers of Ji Infantry en masse should form the backbone of more advanced Chinese armies, using their long reach and propensity for splash damage to defend against cavalry and to obtain melee infantry supremacy in the field.
Having said that, Ji Infantry are powerful, but they are certainly not invincible. Like most early to mid-game Chinese infantry, Ji Infantry units are still unarnoured, so while they are devastating if used in melee, it is obviously wise to NOT engage them head-on. Any ranged unit - be they War Chariots, Skirmisher Levy, or even Steppe Nomads - will be highly efficient if used to take them out, since Ji Infantry have one weakness and that is their inability to respond to ranged attacks. Obviously, if you are the one arming infantry with ji polearms, the you should endeavour to defend them from these threats, preferably with missile units of your own or artillery where possible.
Pre-20th century Chinese armies predominantly used four classes of melee weapons, which were the Dao (sabre), Qiang (spear), Jian (sword) and Ji.
Of these, the Ji was a weapon unique to China, being a polearm with a steel or bronze tip mounted on the end of a long shaft, next to which is attached a curved blade. Because of the attachment of the curved blade, the weapon can be used to both stab and slash.
The Ji was first used during the Shang Dynasty (17th-11th centuries BCE), when it was made of bronze. It was popularized during the Eastern Zhou Dynasty (770-256BCE), when it was made of steel instead. Conflict with the nomadic tribes of Mongolia and Siberia soon resulted in the Ji being replaced by Qiang-type spears as well as other polearms.