You'd think they'd at least make the Wood Elves something of a threat beyond 'eeeh its gonna be really hard to take these guys out because attrition.' I've never seen them do much more than attack Parravon, and Bretonnia gets to live pretty quietly all in all. Some games they don't do much beyond unify Bretonnia, some games they manage to conquer territory in Marienburg and Nordland...but I really feel sorry for Mousilion, goddamn. One single little territory, and they seem too afraid to ever attack their neighbors and usually get eliminated by the midgame. Though I have seen Mousilion take Lyonesse and L'Anguille once, and for a brief moment they had wolves and knights instead of skelle and zombie spam. But otherwise they're like the Wood Elves - more of a hassle than a threat.
I'd hoped with a setting like Warhammer the Total War game would include a lot of threats from a ton of different angles to juggle, but it seems more like there's always a set path to fight through and not many enemies, and the higher difficulty settings just mean the major factions confederate and get their goodies faster.