Not necessarily, but I would question the curriculum that is being taught. It might be watered down or diluted.
For example, take this website which pretty much "sells" JKD certifications
Bruce Lee Jeet Kune Do Jkd Martial Arts Instructors DVD training
If that was the instructors basis for his teachings, would you consider him legit. He may be a good teacher, but a good teacher who is teaching inferior curriculum doesn't mean he is legit.
I would also suggest that if he was a good teacher, and was looking to improve himself, he would try to always seek more certifications, curriculum, ways to become better and provide a better product for his students. In fact, isn't that one of the principles of JKD? Not to limit yourself to one style or method....