Might want to be a bit more careful with the words "entire map" as no nomadic empire has ever conquered all of Europe (or all of Asia).Actually, nomads in RL in the Middle Ages steamrolled the entire map several times
When conquests like those of the Mongols occurred, it wasn't because nomadic horse archery was an inherently superior method of waging war; though it did have advantages. It was due to a combination of many factors. A large factor was that most of the nations they defeated had unstable governments at the time.
Additionally, since you mention RL, Bannerlord's setting is based on the 600-1100 time period in Europe, North Africa and the Near East, and in this time/place, the Avar and Khazar Khanates, some of the inspirations to the Khuzaits in Bannerlord, actually got wiped out. If we wanted to follow real life to the letter, the Khuzaits should actually be the first kingdom to die, rather than steamroll.
Not that I am actually suggesting that. I'd prefer a game where every faction is reasonably balanced, except Battania is a bit weaker as the "underdog" faction, and the Empire factions are predisposed to destroy each other.
I wouldn't call losing a third of your health from an arrow while wearing a full set of lamellar, like I did just yesterday when fighting Khuzaits, "minor wounds"I can pass through a whole mass of Kuzaits with a couple of minor arrow wounds... but passing through a clump of Faris... each with a one hit javelin missile... scary stuff.