It is very hot and humid in August. But the parks are less crowded, especially the last two weeks of August. That's when we go.
Just plan for the heat. Florida is hot most of the year. In fact, August is just a few degrees warmer than May and June. Nevertheless, if you tour all day, the heat will take its toll. The good thing about August is it is less crowded but the parks are still open late. So we always tour the parks in the morning, go back to our resort around 1 or 2 to rest and swim, and then return at night for dinner and a couple of attractions or a closing show. We usually stay about two weeks and during that span, we typically limit our afternoon tourning (between 1 and 5) to two days. Some afternoons are necessary to see the parades.
Any way, August is a great time of year, IMO.