I'd say AI lords in Stronghold Crusader are pretty tough! Considering the fact that making a smart AI for a Stronghold game is way harder job to do then in Age of Empires for example. In Age of Empires it only needs to build up its colony/town, build an army, defend themselves when attacked, and attack when has enough troops. Simple as that. In Stronghold it is far far more complicated (no need to explain why), and considering that Firefly has done really good job in Stronghold Crusader, although we can find some ways how could they be even stronger, such as - make them so they would put more troops onto their castles.
But, if you give them advantage, take less gold for yourself, then they grow strong enough to pose quite a challenge. Yes, we can easily outsmart them, their attacks are often frustratingly inefficient, but if you decide not to play dirty on them, then it can be an interesting fight.
Also, their attacks are obviously much more effective against human players than against other AI players. Try watching them fighting one against the other, and you'll see tons of situations where one could finish the other, but decides to walk away just to destroy one more woodcutter. That doesn't happen with human players so much mostly because AI lords have better multitasking than us, and when we build, we build, and when we fight, we fight. More or less.
Having that in mind, I'd give them 7/10. It is far from perfect, but yet, Firefly has done an outstanding job with AI lords.