Having played M&B Warband numerous times ( in different games), I found out that there is no "supreme" faction in this game, neither a "weak" faction. I've seen Sarranids own the Khergits and Rhodoks a lot of time, and I've also seen them fall down against those two foes. And it is the same for pretty much any faction.
Usually tought, the Sarranid do just fine, even without my help. They keep on going in alternate wars with the Khergits and Rhodoks, manage to claim Narra and Unzdaq from the Khergits, while they keep taking then losing Almerra and Weyyah from the Rhodoks.
My longest game as a "neutral" character was about 482 in game days, the Khregits wipped out Swadia completely, while the Sarranids managed to take Narra and Halmar, tought they lost Weyyah castle to the Rhodoks.
I created an independent faction in Khregit territory then, and took all of their Swadian lands, while the Sarranid helped take down the last Khregit lands, I wonder how the war would have ended if I didnt make my claim then...