Yes they can be depending on how bad the weather is. Fantasmic is often canceled due to weather because of the nature of the show. The other dispays less often but it still happens.
I agree with safetymom twice last year we tried to see fantasmic and it was cancelled. There was not a drop of rain but plenty of lightening.They even closed the boat from MGM to Bcv we walked back .
They are very cautious when rain and/or thunderstorms hit. Several years ago, we attended the Very Merry Christmas Party and the first parade was cancelled because the streets were too slick. They held another one later that evening. And they do stop the boats from running at the mere hint of lightning, which is prudent. I think Orlando is the lightning capital of the world...best to take it very seriously.
Wishes was cancelled last tuesday due to rain/lightening. It was kind of cool though--they had to shoot off the low level shells, rather than keep them in the tubes so we were able to see them on the way back to ttc about 1.5 hrs after park closing.