As I wrote, that is my personal opinion. I took some shots. Not hundreds, just some.
I focused on high contrasted things, like an advertisement of nikon ( :-) ) and on other, more low contrasting things. In my opinion the AF of the 5D3 was as fast as the 6Ds. (subjectively!). And it had its problem like the 5D3s on focussiong on an plane, lowcontrasted thing. Even in changing the AF field manually ( that the seller did not like, when i performed this)
And, for me, the AF-points are too much horizontally orientated.
The Nikonians there told me, that the 6D is just "a shot out of the hip to react on their overwhelming camera"