Not really wanting to get embroiled in an argument, but do want to add here that Apocal is selling the straight goods regarding real world history and the impact of Steppe Armies on their agriculturalist neighbours. Some of the counter arguments are pure speculation or just wrong.
To add a bit (of the history; I think this game has the best representation of HA to date, but I'm sure there's more work to do re: other troop types and balance in general...):
It wasn't just the Mongols - Steppe Nomads (and pastoral nomads elsewhere, to a degree) were almost unstoppable until armies started fielding reliable firearms and artillery.
The Chinese and Russians basically closed the Steppe in (IIRC) the 18th century - until then the horse archer ruled.
It's a visible trend toward the supremacy of firearms - the later Steppe origin empires (Timurid and Mughal e.g.) had to incorporate more and more firearm infantry to achieve the same thing. The Native American confederations on the great plains could not manage it, because <guns> (and the effect of European epidemics, but when they fought back - guns).
The Steppe peoples pretty much invented real tactical and strategic mobility - the bronze age empires got their chariots from the Steppe, not vice versa, and later on probably stirrups. The Huns might have been displaced Xiongu, or a closely related culture. Greek 'Centaur' and 'Amazon' myths are probably reacting to the Scythians, etc..
Both Han Chinese and the Byzantine Empire came up with very similar military counter strategies - as alluded above, well led infantry and archers / xbows with mobile cavalry wings - it worked when it worked, but it was invariably cheaper financially and politically to pay tribute / divide and rule or hire 'the other lot' as mercenaries and hope they didn't decide to take over.
Attila, Genghis, Sundry Turks, Tamerlane et al appear to have learnt everything they needed to know about siege craft any time they needed to. Genghis and his successors co-opted / hired Chinese and Persian specialists as a matter of routine (+ the pragmatic strategies mentioned in other posts - they really did flood out cities and use frozen rivers as highways).
They could all fight on foot when they needed to - they often dismounted and 'forted up' in ring formation if things were going poorly - the Mongols did that at Ain Jalut (the Islamic victory mentioned above) - once their horses were blown and the strategic situation precluded resupply and proper mobility.
Many of the big Middle eastern empires started out as steppe peoples at some point, and retained the strong cavalry focus culturally and militarily; Sassanid, Parthian, Etc. At times India. At times Russia. The Chinese Yuan Dynasty. Europe, Japan and the Vietnamese felt the fear more than once. The Mongols probably hit their logistical limit rather than any military nemesis - they couldn't hold anything much beyond the Levant and the edges of western Europe - but they could and did beat the living c**p out of most of the armies they met along the way. They certainly also used terror tactics, but if that were all she wrote, then the IRA would be ruling in Westminster, and ISIS would have an actual Caliphate up and running.
It wasn't all horse archers - the whole nomadic lifestyle lends itself to mobile warfare and effective battlefield communications - if you could live like a steppe nomad, you were going to be some use in a steppe army (or in earlier times 'big raid') - you have to be able to communicate and react in a coordinated way to herd livestock effectively (Westerns and stampeding cattle...) - and as livestock is much more mobile than, say, wheat, you have to be able to mobilise quickly and effectively if someone steals it from you. Then proper oceanic shipping and good guns, and things change.
Personally I think the history is important in relation to Bannerlord - the game does the feel of Medieval warfare better than any other I've played (that isn't the same as saying it's 100% - but it feels right more often than not) - In a world casually informed by Hollywood, I think the developers deserve some kudos for their efforts to create something that feels right for the period they are presenting (yeah, yeah, I know it's a 'fantasy' universe, it's just a fantasy universe based on early medieval Europe, the Med, and the Middle East). As a comparison, HA in the total war series do all right at the tabletop wargame level, but the micro is a complete pain, and there is no sense of the sheer fluidity of HA in combat.
Ignoring exploits and the AI deficit in some other units, you can counter them with historical tactics in game if you want to - that's 'historical' not 'Hollywood' (not knocking Hollywood, just not usually a good source for historical info). I usually stick shield infantry in wall or ring protecting archers in loose and hands on managing cavalry - the shields in a defensive formation cut down casualties and act as an speed bump, the archers get kills, and the cavalry if managed actively distracts them from the archers and doesn't get to tangled up in melee. It worked, and in the game it works, and the only real micro is periodically recalling your cavalry before charging them in again. I like to fight hands on, so I often just get them to follow me.
A final point on HA aim - along with the comments above re: targeting armies rather than individuals (which is correct - a bow isn't a 'sniper' weapon until modern compounds) it is worth noting that HA is not a long range sport - Ideally you get in close and shoot at point blank range whilst not getting quite into reach of the other side's weapons - that's why only some people's develop the skill - it's a lifestyle thing - you got to be able to ride like as nomad as well as shoot like one. Give a Russian Peasant a reliable gun and a few spare rounds and, game over.
