I've done many parties under windy conditions. My rule of thumb is that wind (including gusts) up to about 15mph is tolerable. Anything over that and it might not safe. But, I'm thinking the tolerance should even be lower. Not that the inflatable screens can't handle the wind. As long as they are staked well, there usually isn't a cause for concern.
The problem is that the wind makes the projection material stretch and bulge. This creates two problems for the viewer. First, when the screen is bulging (either in or out) it creates a sort of psychedelic look. It's kind of funny and strange at first, but quickly becomes distracting. The second problem is that when the screen bulges too much, the image goes out of focus. The projector is set up and focused on the plane of the screen. But once the screen bulges too much in one direction, the distance between the projector and the screen changes. This can throw the image out of focus. Also very distracting to the viewer.
If there are only one or two gusts over the length of a movie, then no big deal. But, how do you know this going into the movie? If it becomes too distracting, it actually leaves a bad impression with the viewer. Most people won't hold you responsible, obviously you can't control the weather. But, the question is, would be the wiser decision to recommend postponing an event if there is a risk of this happening? And, I don't know where I would put the cutoff, but I've been thinking that 10mph might be better.
Here is a picture of a screen bulging. This is about 8-10 mph gust, so you can see how far it really can stretch.