Moon of nightmare
For an illusionist, imagination is key. It was thanks to that that one could compare a good illusionist with a bad one.
A good illusionist will be able to create new spells at every single of his move. Be it fake or real. And thanks to this unknown factor, the enemy would be tired to discern which spell is true and which spell is false.
While a bad illusionist would be characterized by his bad imagination. Without imagination, the latter won't be able to do much in a fight since the spell he will use would be those of other classes that he would have seen in his life.
So being able to create a spell on a whim just by thinking of it was an achievement in itself.
When creating something, one needed to imagine every single detail of it. So being able to do that during a fight without much delay, was something very complicated.
Though, being able to do it would bring more benefits than drawbacks.