Pure speculation here - at least until they reveal more of the combat mechanics - but here are some historically-based possibilities:
1. As stated above, more stability/loyalty, because the Mongols managed to endure centuries longer than the Huns
2. Larger Army sizes, to represent the far better Command and Control of the Mongol armies compared to their predecessors (ALL their nomadic predecessors, apparently)
3. Better ability to include units from conquered Factions or Minor Factions, which would be representative of the fact that Mongol armies included Chinese Siege Engineers and gunpowder-users as well as mounted units from other Central Asian factions they had conquered. The Hun's own 'subordinates' as mentioned previously, turned on them as soon as they could, whereas the Mongol subordinates for the most part stayed loyal.
4. Possible ability to exchange some of their own Emblematics for those of a conquered Faction, as the Mongols did when they settled down and became the Yuan Dynasty in China, the Golden or Great Horde in southern Russia, or the basis for the Timurids or Gazhnavids. I think this would be way OP, though, so it's a real Long Shot.