In FotS, you're looking at more like a fifth of the map being on your side before realm divide. If you allied with someone beforehand and keep them happy, you can effectively conquer half the map with little difficulty. In TWS2 you could capture roughly 20-23 provinces before triggering realm divide, which is roughly a third of the map. The first is that realm divide happens much earlier in FotS than TWS2. It's much more rabid in FotS than Shogun 2 though, for a couple of reasons. Some of the frustrations from Shogun 2 are present in FotS, namely that allies have an extremely high chance of betraying you, even if they're vassals you liberate from factions that they hate.
