Not in all situations where the other two may get it. But not all subjects move at high speed, and then it's very adequate.
I tried my luck at moving subjects (runners) with a lot of "stir" in the frame (several runners) in low light (aka pitch black) in bad weather (rain and storm) using the 1DX equipped with the EF 70-200 mm f/2.8L IS II USM and on top the 600 EX-RT powered by a CP-E4 battery pack and on the side the GP-E1 GPS unit. Here's one of the results. Now consider that before the flash fired, there wasn't much light except the headlamps on the runners, and that's not much help for the AF when you get the light straight into the camera instead of illuminating the subjects.
Of course I don't know how well a 6D would have performed in the same situation. What I do know is that the 7D can do the same thing, but it's easier to get keepers with the 1DX. Here's an example using the 7D, equipped with the EF-S 17-55 mm f/2.8 IS USM
, on top the 580 EX-II
powered by a CP-E4
and below the WFT-E5B network grip
with Kensington Bluetooth adapter
connected to a Wayfinder GPS
puck. I mention the extra equipment only since it may affect the available processing power in the cameras.
This is with the 7D.