I normally wear running shoes (very good running shoes), but there are times I switch over to sandals. If I’m going to a water park, if I’m taking out a boat, if it’s really raining and there will be running water and deep puddles (like around Spaceship Earth), or if I’m sure I’m hitting wet rides.
But the last time I was there, I developed some nasty blisters at the extreme end of my heal as I was heading back to the front of the park from Splash. My sandals are Columbia Sportswear (like my shorts) and they are well made. But this points out that there is a potential downside.
I would not wear flip flops in a crowd environment.