I played as scotland and kicked some serious ass.
the main reason why they are good in the Campaign game is that it is easy to secure the British Isles which you can use as a base to send out armies by sea.
BUT the main advantage is that they are undefeated in a defense cuz of all the pike man units.
There are also very good at attacking cities/castles for the same reason.
But they are terrible in the open field cuz thay have no good archers and their Cavalry is not the best.