Even the best allies will slowly decay after the realm divide eventually.. but that just means you need to work hard with your relations to kepe them as allies. Give them gifts, marry out your daughter etc etc.. these are all good things to do with allies to help build an even stronger relationship.
Does Fall of the Samurai have realm divide?
The first is that realm divide happens much earlier in FotS than TWS2. In TWS2 you could capture roughly 20-23 provinces before triggering realm divide, which is roughly a third of the map. If you allied with someone beforehand and keep them happy, you can effectively conquer half the map with little difficulty.