The major limitation of using video in the classroom is that it allows students to be too passive.  If you just put a film on and have the students watch it, you are not allowing them any sort of an opportunity to interact with the lesson.  Even if you are just lecturing, which teachers are encouraged not to do, students can ask questions.  But with a film, the students are just sitting there, typically in the dark, and passively watching the film.  At the very least, if you are using a film, you would want to have some sort of a study guide with questions that they should be able to answer as they watch.  Otherwise, you run the risk of having students pay very little attention.