My youngest does not do well with "creepy". She turns her head during "Walking Dead" commercials or the like, wouldn't look at Darth Vader at DHS, etc. At 6, she found Fantasmic to be just downright horrifying. Fantasmic is essentially "Mickey's Nightmare". If that's the kind of thing that scares your child, I'd steer clear.
On a side note, nobody else in my family really liked it either. The water projection was kind of cool for the first minute, then the novelty wore off. Honestly, the pre-show festivities were way more fun than the show itself.