Sergeant at Arms
The problem is that Khan's Guard straight up overshadow melee cavalry at their own job. Banner Knights, Druzhinnik Champions and Elite Cataphracts don't even have the melee punch of the Khan's Guard. What's the point of noble melee cavalry when they don't fight as well as a literal horse archer.
I guess that all depends on how they use the skills they have. A Banner Knight at full charge with couched lance should drop any unit they face - including a Khan's Guard. That's how they should be deployed. And given the gaming paper scissors stone logic... they have the OP charge, but should be weaker in melee. Do they couch their lance every time they charge? Unlikely. An Elite Cataphract should be able to go toe to toe with a Khan's Guard thanks to their superior armour, and shield - and again, should be able to drop any unit in the game at a charge with their lance - but do they charge home every time you tell them to? If you're lucky and they don't ride off in the wrong direction.
Banner Knights and Cataphracts have an objectively more lethal primary weapon than the Khan's Guard... Their lances will one hit any unit in the game. Including the player with the most OP armour - which is great. Khan's Guards' bows are unlikely to one-hit either of the other two - at a charge they can't come close. On the other hand, Khan's Guards have an objectively more lethal close quarters secondary weapon - their glaive is more lethal than the other's swords. There is a balance to be found in there. Any player kitted out like a Banner Knight would drop a Khan's Guard in single combat. Because the player would couch lance and charge home - even if they weren't confident in targeting they might first drop the horse and circle back.
This is a problem with how Banner Knights and Cataphracts use the weapons they've been given - the glaive is better than the sword that Banner Knights and Cataphracts fall back on if their charge fails. And the charge fails too often. But I don't think this means that Khan's Guards are too overpowered, I think it means that other units don't play to their best features well enough. As a ranged unit alone, Khan's Guards suffer from the same failings all Horse Archers do - they only occasionally hit their target. This is a distinct disadvantage relative to crossbows for example.
On the other hand... Don't get me started on Druzhniks. They are legitimately flawed.