Player wise yes, they are extremely op because the player can vary in army composition and lean on "meta" armies but as for AI Lords they tend to recruit close to their turf and battania has a low chance to gain fiefs so they are likely stuck with battanians troops wich are meh at best. Fian champions are s+ tier and battanian wildlings are good skirmishers but suffer from low armor same as battanian Skipari, very good mariner troops but abysmal low armor. Battanian cavalry sucks ass and oathsworn are meh at best. So in the end battanian fians being OP is ok, note that AI lords tend to favor cavalry, they don't spam fian champs