One thing I noticed. If you decide to choose a pre-made faction, they always get some kind of hidden bonus.
So lets say I join Nords. They'll kick ass most of the time.
If I join swadia, they'll kick ass most of the time.
So on for other factions.
But whenever I am not nords or swadia (and I should add, all I do when testing this is sit in a city for months and months getting fat and corrupt) or whatever faction...they don't tend to do as good.
So whatever faction you join, while it IS a bit random...but if you join a faction, it gets some kind of bonus to do better than average. Even if all you do is sit in a city, eating up food and hiring prostitutes
In 1257, the hidden bonus seems a lot less or not at all, as even if I join crusader states or welsh or some other faction if I don't help them out then they have as much of a chance at winning/losing as other factions.
But in vanilla, the faction you join as a higher winning chance compared to other factions.
This was tested in vanilla, floris and A New Dawn. All 3 had the same experience and all use the same factions/map, the worst of them was A New Dawn which the winning chance was much higher than vanilla for the faction you join.
The mods that this does NOT happen (that I have played, if it does, the bonus is not noticeable compared to vanilla) is 1257 ad and 16th century mod.
Not sure if anyone noticed this little game mechanic feature or not. I know most make their own kingdom (which the AI always targets you no matter how big you get), so I don't bother with it)...so this is more information for people who like being a vassal and don't want to be a leader of a faction.
And, yeah...I wasted A LOT of time to figure this out
(I should also add, that isn't to say joining Nords suddenly makes them win. They just get a higher chance of winning if you join them, whether you do battles or sit in a city forever)