You disrupt the flow of chakra in your opponent's brain when you use any sort of genjutsu. Whatever the medium may be, your opponent needs to know your location in order to preform the genjutsu in the first place in order to cast the technique.
It's like saying you use the fireball jutsu at your opponent in the dark and you don't know where they are, you're directing your technique blindly so if nobody knows Zabuza's location within the jutsu then he's good money when it comes to gen.
If not then he's SOL unless he can disrupt his own flow to release genjutsu, however with his status, I believe him capable of dispelling genjutsu. Maybe he's just not the best at casting or recognizing it right away idk. This is my view on it.
