For some reason, Incendiary Arrow provide more damage boost than it should to the Rus Horse Archer, making the Horse Archer performing much better than intended at late game.
Elite Horse Archer fully upgraded with Black Smith tech will have 17 damage, when Incendiary arrow is researched, it should be: 17x1.2=20.4 (which is rounded down to 20). But right now, the Horse Archer do a total of 23 damage after all Black Smith tech and Incendiary Arrow tech is researched. I don’t know where the formula is wrong but the Horse Archer gain 3 extra damage and are doing much better than it should be.
Edit: Incendiary arrow also slightly reduce the reload time of Horse Archer from 2.0 to 1.88 in the tooltip, i know their atk speed is also wrong compare to the tooltip