There's no such thing as good or bad in M&B, it's all shades of grey.
That's always been the case, even in the real world, but the retarded Western media always
has to have a 'good' and 'evil'.
Anyway, here's a general overview-
Nords: Excellent high-tier infantry, but have deficiencies in ranged support and cavalry (aka none). You probably need to recruit mercenaries or non-faction troops to make up for this, unless you really enjoy playing pure Nords. The Huscarl is the benchmark against which all other infantry is compared.
Vaegirs: Known for their archer tree. Vaegir Marksmen are absolute bloody murder; but they don't really lack in other trees. Decent infantry and cavalry.
Swadia: The heavy cavalry are bloody awesome. Swadian Knights are extremely ****ing good, even spears and pikes don't do much to stop a massed charge. No deficiencies in ranged or melee, but the ranged tree usually uses crossbows.
Khergits: Patterned after Parthians and Mongols. Almost every troop is mounted, though I'm not so sure about the basic Khergit Tribesmen, and have the best light cavalry in the game (the Khergit Lancer). Khergits are whined about in almost every sub-forum, so that kinda gives you an idea of their ability.
Rhodoks: The noob faction. They're somewhat of a 'peasant rebellion'. Their melee tree is real bad.That said, their ranged tree is pretty decent (Rhodok Sharpshooters are excellent crossbowmen), and they seem to breed like rabbits. If you do not wish to tear your hair out in frustration when you play this faction, Fei Dao's Rhodok improvement mod is highly recommended. It turns the Rhodoks from abhuman peasant scum to actual contenders for Calradia.
Mercenaries: If you can, you usually want to avoid recruiting from here. IMO, the crossbowmen are suck. The infantry tree and cavalry tree lands you with half-decent troops, but they do seem to cost more in upkeep.