Okay, I haven't played in a while but here are my various faction likes (and dislikes) in a very short form
Based more on pure feel and fun than anything. I've played every faction multiple times for many hundreds of days and never had any real problem doing well with any of them either strategically or in a tactical battle.
Sarleon - fun to play, love the feeling of starting surrounded; their elite troops always seemed a little under-powered to me but at least they all looked good
Ravenstern - hands down my favorite faction, my biggest problem is purely of of geography...the inland sea and no boats is a major PITA
Fierdsvain - always seemed a little OP to me, a little too like playing on easy to make them do really well; ridiculously slow horses...
Empire - sorry but I cannot stop laughing at the bad faux-romans on horses, elites with pila horribly OP but their whole army is just fugly
D'Shar - never liked them; they do well enough but they don't really seem to 'fit' with the others visually or culturally (It's like you have the french, english, vikings, psuedo romans and then suddenly Mongols...with no Poles or Germans in between)