I don't know what the fuss is over. I was nailing keeper after keeper today with my supposedly horrible 6D and my completely unusable Tamron 70-200.
Which af points were you using?
All modern af system are usable, the differences should show a) when the tracked object is to the top/side (no af points on 6d), b) when the contrast/light is low (only up to 0.5lv and no outer cross on 6d), c) on difficult movement patterns (i.e. not simply something like a wave moving constantly in one direction).
However I expect the 6d to do just fine for me once I got it, I recently got frustrated by my 60d again because the af fields are just too far apart so it's center point only for tracking - which works, but at least on 6d the points are closer together so multipoint should be more usable.
Center Point only, continuous shutter mode, one shot AF mode(because the Tamron lens does not function well in the AI modes)