I've played this whole game under the impression that swadia AND the vaegirs suck,
as I see it, the nord have the best on foot melee(huscarls) making them best in sieges,
the khergits have the best archers(even if some other archers hit harder or more often, you can easily ride up to most of them on your horse and kill them somewhat easily... when the archer is moving rapidly on a horse himself that changes everything, it's hard enough when its a melee unit you can't catch, it's even worse when they can hurt you from your position)
the khergits are also the best ground units because cavalry has a massive advantage over things that are not cavalry
and that leaves the rhodoks and sarranids, now I don't know anything about the latter(sarranids) so I can't judge,
and the former(rhodoks) has spearmen, which are basically the best things against cavalry besides cavalry and debatably besides archers which doesn't rank them too high on the whole vs cavalry scale, but they don't seem that useful in sieges(which they share with cavalry)
swadia and vaegirs seem to be the only factions without a unique hook/edge that doesn't make them completely useless, well roundedness is useless in M&B as long as whatever faction you are at war with isn't warring with the faction that has the best of the troop type you currently want/need
now if you manage to anger the nords and the rhodoks and the khergits, then you better be swadian or vaegir because that'll be the only time that well-rounded units is actually useful
IMO.
P.S: now that I think about it, a minute or two later, most of this post was colored by the fact that I mostly used Melee until.... yesterday night(now I'm a huge fan of archery) so things may differ if you use archery instead of sword, shield and lance or if you use throwing weapons(for some reason) or polearms off of horses(but that sounds pretty stupid to me)
