If your critical resources are next to each other, Byzantines need to commit harder to secure them, and Mongols can deny that faster because of double spearman production in dark age.

Spearmen can bait out the Hippodrome Triumph activation, getting value from forcing the ability.

Keshiks are armored and can withstand Town Center fire, so they can dive.

The Khan Hunter's attack range buff reduces the difference between Mongol archers and Byzantine longbowmen.

The extra 10 HP on Limitanei is a buffer, but against Mongols, the double production means they can outmass you, making the extra food cost particularly painful.