The 5D3 does horrible in harsh lighting with a lot of overblown highlights and shadows, such as outside at noon on a hot summer day. The RAW files aren't as easy to work with as the 1DX files are in those situations, I agree.
I'd hardly say the 5DIII is horrible in harsh lighting. Harsh lighting is horrible, generally, yes, but the 5DIII handles it as well as you could reasonably ask it to. No camera is going to fix bad light.
Here's an example I was playing around with last week. Obviously, it's Arizona, the middle of the desert. And it's midafternoon. And I'm in the bottom of a steep ravine. I can't imagine anybody seriously looking to do anything pretty in harsher conditions.
This one isn't going in my portfolio, and it's not the shot I was out there to make, but it's a far cry from "horrible."
It's also basically straight out of the camera. No lifting of shadows, no HDR, no noise reduction...just white balance and exposure normalized with a ColorChecker passport, scaled, and sharpened for the scaling.
I took a three-shot auto HDR bracket; I always do for landscapes because it's so easy. But I wouldn't use any other than the normal exposure for this shot anyway. No need; the camera has all the dynamic range I needed for this shot and then some.