I found the Horde to have a little more variety in XL. They brought units of Korean spearmen as well as Mongol Foot Warriors - tough archers/swordsmen.

I agree that they should have much higher-rated generals and high morale. However, I know that many people find the endless battles a bit of a chore, and having very high morale for the Mongols would make this worse.

Maybe high valour as well, so the 'endless' and big number battles wouldn't last long - the player would always lose until the numbers fell away?