Anyone who says that cav only armies dominate on the field of battle needs to sit down and study some history. Mongol armies were strategically, logistically and tactically well ahead of their time. But when it came down to the actual battle, mongols could not break western european heavy infantry/cavalry formations unless they had days worth of outflanking/harassing etc.
The khuzait racial trait and their cav only armies emulate the advantage of having 5-6 ponies per soldier. The autoresolve giving a flat 30% advantage to all cavalry does not do it justice and basically lets all tiers of khuzait have the advantage AI vs AI. The fact that armor is useless and range is incredibly strong in BL their horse archers just become murder machines. So yes, the khuzaits need to be toned done, the autoresolve system revamped, and armor made useful.
Actually, a long standing idea of mine, was to implement some "cons" to the factions as well -- as in, for example the Khuzaits currently in discussion, add "unskilled engineers" punitive trait to Khuzait culture leaders of parties/armies and remove them from being able to use onagers, trebuchets, ballistas, siege towers.. and simply limit them to battering rams and ladders. Also, this penalty will generally lower the building speed of anything related to engineering -- siege camps, battering rams, building in castles and towns.. etc etc..
History-wise, the bane of steppe-armies were siege battles, and their inability to take over well-defended positions.
Even the Mongols took 44 years to defeat the Southern Song dynasty in China, spent something like 6~7 years in the siege of Xiangyang, and only after getting their hands on trebuchet technology from engineers from the Middle East, they were able to push on. From this experience, the Mongols began including a considerable number of infantry and engineer corps in their armies, and they became the masters of siege battles as well, but before that point, even the invincible Mongols were extremely poor in siege battles and their basic strategy was to "just wait it out."
In the case of Manchurian Jurchens -- the founders of the mighty Qing Dynasty -- they weren't able to make it past the Shanhai Pass fortress, and the founding leader Nurhachi died after having been injured by the defensive artillery from the fortress. The Jurchens entered mainland China only when the garrison of Shanhai Pass basically gave up and surrendered due to certain political reasons.
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So, if we translate this into the game, it would mean that the Khuzaits would be faster than most, extremely capable in field battles, but their sieges will always take much longer than other factions. Their sieges will always be exposed to enemy defensive artillery, without proper means to retaliate, and all they can do is simply break down the gates and storm the walls on ladders.
I think a range of similar "cons" can be added to a lot of other factions as well, to both increase the 'flavor' of each faction, as well as implement necessary balance. In that sense, the Khuzait can be a dominating force on open field battles, but always having trouble in actually expanding territories through siege conquest.