The Norse (and basically the Pagans in general) all start with event troops. So I mean, they're a known factor, it's not a "random levies they can't support".
Well, it technically -is-, but without 'em, the pagans would be fucked over within 2-5 game years.
Instead they fuck everyone else over. Not a perfect system.
No, it's not the Norse, themselves, it's the "peasant" revolts.
The Norse took over a county near the south of Ireland, and then the religious/cultural unrest and other factors caused the revolt chance to hit 90%, and a revolt took place, spawning in 7000 troops from nowhere.
This "rebellion" had leaders with 20+ military stat, and was hostile to everyone (but at the time, I could just get out of the way,) even while conquering most of what the soon-to-be Kingdom of Denmark had taken from England/Ireland. I tried to lead a strike against Denmark, taking advantage of the "cover" the rebellion provided, but trying to take anything during this revolt meant that ultimate doom stack of 7000 troops would come my way to take their castles back.
Once they won their rebellion because NOBODY could stop them, they declared themselves King of Ireland, made the Kingdom of Ireland, (when they only had 2 of the counties of Ireland, and I had 6 of the 7 I needed to make the kingdom!,) and then started annexing everything else.
And Ireland wasn't the only place that happened - Scotland was getting clobbered by the pagans, so they had a 7000 troop super army spawn in from nowhere as a "peasant" revolt in a country whose
actual kingdom was only capable of mustering 1500 troops on a good day. West Francia had nearly obtained a full Frankish Empire before being ripped apart by two simultanious "peasant" revolts that spit out triple the number of troops on its lap than any of its non-cheating neighbors could muster.
The "peasant revolt" system is just flagrantly broken and completely unsuited for that time period. Since when do "peasants" have thousands of knights at their disposal? Where were they keeping them? The simple answer is that these aren't peasants, they're obviously just generating some mercenary doom stacks to create an army, and don't give half a damn about game balance when they create these things.
In Old Gods, the castles and such have no upgrades whatsoever, so I'm capable of mustering about 400 troops per county I can control, but these guys spawn in troop numbers that are obviously geared towards giving an established kingdom from 1066+ trouble. It's flagrantly unbalanced.
Again, I thought this game had had years of patching to balance this stuff out? These things like mercenaries and peasant revolts are blindingly obviously unbalanced, I can't imagine they wouldn't have figured this stuff out if they had ever bothered playtesting their game at all.