Islands also has a lot of release valves. If you see rain in the forecast you can just go next door to Studios where almost everything is indoors, or take some time off to grab a drink or a snack at CityWalk. For the foreseeable future Epic is off on an island by itself like Animal Kingdom (another park that almost completely shuts down for lightning).
I had been thinking of buying another last-minute preview ticket to enjoy the low crowds since I had such a great time on the 27th, but looking at that data it seems like I really lucked out with my choice of date, so I'm glad I didn't press my luck. Here's hoping they have their act a little more together by Memorial Day