Yes it has been mentioned. They just don't know how to "balance" the game properly.
A couple of thoughts...
Snowballing isn't the problem,
one faction consistently snowballing is. Over the long term, one clan moving to a new faction, or a single city switching hands, creates potential for a snowball. It changes the logistics equation. A faction has a potential 4 or 5 fewer parties and 2 or 3 settlements from which to recruit. This small advantage is where a snowball might start.
But the hard part, is preventing a situation where just one faction always takes that first city, or gets those few extra parties that disrupts the status quo and turns that first advantage into a snowball, wrecking them map. This could be caused by something as small as a few damage percentage points in a single weapon that a faction has. It really is butterflies in Africa.
So to fix snowballing two things have to be addressed:
1. Whether only one faction is ever doing it (and solving this without making that faction weak, or by replacing it with another faction that snowballs). And in so doing, discovering why one faction is snowballing - is it that bow they use?, is it the extra armour their archers have? is it how their faction benefits uniquely from a particular law change? is it a faction trait or the laws it starts with? is it the balance of the village economy the faction has access to? is it the unique traits of a faction leader that lead to differing outcomes for simulated fights early game? is it the spread of traits the faction's starting commanders have? is it a gender/reproduction issue that leads to imbalance in characters or child traits? is it that a faction's economy is adversely affected by randomised bandit predation? - and we could go all day with this.
2. is it that any faction might do it, but it might be different every time? If this is the case, the units, leaders and traits might be in balance, but the simple math of how small advantages in numbers when land or people change sides impact on the game might need to be addressed - usually this is done by making it harder for factions to control or govern territory they take (this is the solution used for Total War games, and is increasingly what we're getting with Bannerlord)
The hard part, is that we all play certain ways that upset the balance of factions.
We cause snowballs where they might not otherwise occur by stealing party leaders through marriage, by raiding inconsistently, by changing the economic base through our trade, by changing bandits locations, then by taking castles, cities, recruiting clans - which all changes the balance.
And we might be playing a single game for weeks, or months, so judging whether balance is correct, on enough different human impacted scenarios, with enough different playing styles, to be acceptable, takes time. A lot of time. And then you might find you've changed something too much, and have to go back and try something different.
In summary, creating complex systems is hard. It takes time. Bannerlord is trying to be a little bit of a lot of different game types. It is a little FPS, a little RPG, a little RTS and more. Most games focus on one or the other of these styles because it is hard. So I'm ok with them taking their time, and helping them along the way.