I tried added six new factions, once. Only three made it in, The Kingdom of Derpland, the Derp Provinces, and the Lands of the Flying Derp.
Imagine my surprise when their lords rode out, five thousand men each, and starting destroying absolutely EVERYONE! The D'Shar got destroyed, the Fierdsvain disappeared, the Jatu got obliterated, and, as the Kingdom of Ravernstern fell down to it's last city, Ravenstern, they all disappeared. No garrisons in the castles, no men left behind, they just rode out to the South and have been lurking at the bottom of the map.
I leave that save file untouched, it's scary to think that, randomly, a party of 3-5 thousand men moving at around 10-12 speed could jump me and slaughter me.