They give you the flexibility of turning your camera into a brick. The Kenko Pro DGX 300 1.4x locked up my 5DIII with several lenses but worked with some, as reported by others for their cameras.
i read that. thought removing the battery fixes it, right?
and i have no 5D MK3.
also did you have the green dot or blue dot or what version of the TC?
It fixes it until you use it again. Then it bricks your camera once more. It worked with the 100-400L. But, I compared the results with the 100-400 without the TC rezzed up 1.4x with PS, and there was hardly any difference. It was the latest version, bought last November. I traded it in last week. I use the Canon 1.4 and 2.0 series III a lot and find them really good. You need these for fast communication between the camera and the latest Canon telephotos. I bought the Kenko to beef up the 100-400L, and it was a mistake.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye