We went to Universal Orlando twice in August 2018 and 2019, and we're considering a Disney trip in August 2020. The weather is terrible, but it's tolerable if you're willing to take breaks in the middle of the day. We usually go to the parks early in the morning, take a break in the afternoon, and go back in the evening. We spend the afternoons at the pool or a water park. The nice thing about mid-late August is that a lot of schools are back in session, so crowds are a little lower. Our schools don't start until late August or early September, so it's a good time for us to go on vacation.
If you don't want to take a mid-day break, you can survive the heat by spending the afternoon in air-conditioned shows and rides. But we get tired and don't mind taking a break.
We've also visited Disney over Christmas/New Years and Easter. The weather is far better during those times of year, but the crowds make me crazy. I've decided I'd rather have heat than extreme crowds.