Guys, faction behavior can change from game-to-game. I think Computica asks which faction is the weakest in actual battle. Given the opportunity, any faction can defeat any other, a victory or two doesn't necessarily mean a faction is weak, it only proves the Player's competence or the AI's incompetence. It takes a lengthy war to determine which faction is good/bad vs. this and that.
As things are now, I'd say the Rhodoks and the Sarranids are weakest. Some other factions could use an update as well (Khergits could use better-equipped top-level troops, Nords could have one more rank of archer, etc) but these aren't so significant.
Rhodoks: their melee troops barely have any notable armor until the highest tiers, which is simply laughable. Their shields are okay when they fight against ranged troops and such, but aren't much help against axes and most two-handed weapons. Also their spears may be adequate versus cavalry, but they are at a disadvantage against short-reach weapons (spears suck when you can't keep a proper distance). Rhodoks could also have some light cavalry. The crossbowmen line troops are okay. Give the Rhodok melee troops slightly more armor and maybe 1-2 tiers of light cavalry armed with light crossbows and spears/maces.
Sarranids: their Archers/Master Archers are okay, and the Mamluks are cool (on-par with Swadian Knights), the Guards do fine as well (though they could be slightly improved if you ask me). But the rest of them suck. Vaegirs, Swadians and Nords all have notably better armors on lower tiers. Also, their basic ranged unit (Skirmisher) starts with javelins, and sometimes no helm or shield ?! Their Horsemen are quite fragile as well for their level. Again, give lower tier units slightly more armor value, a tweak unit equipment here and there (Skirmisher and Horsemen).