Keshiks not being the strongest knight variant, the HRE men at arms get a field day against them, and in theory HRE can just go for a fast castle play against Mongols.

Men at arms trade favorably against most knights in the game, but that's if they're at least equal in resources; with archer support, Keshiks just melt through.

With the Kurultai, archers can do significant damage even against armored counter units, penetrating through armor and taking down men at arms rather effectively.

The healing is just insane for Keshik under a Kurultai. They're just duking it into these heavy tanky men-at-arms, getting healed by the Kurultai, and then getting healed by their own vampiric fighting as well.