The 6D can lose focus during servo tracking, when leaving all points active...because for some reason it wants to rely on the outer points more than the center point...even when the target object is right over the center point. In my opinion, this is a big part of the reason the 6D loses focus lock during servo tracking. It's not so much that it won't "hand off" between all the outer points...it's that they won't hand off to the center point (which is quite a capable point...admittedly the outer points are not very capable on their own.)
If the 6D simply allowed the center point to dominate (when all points are active)...rather than get passed over, the camera would do much better. Certainly in good light with a decent telephoto lens, if you leave only the center point active, and keep that point over the object you're tracking...even especially when the object is moving toward you (less so when moving away)...well in this situation I get about 8 out of 10 shots in focus...even in less than ideal light. It's more like 9 to 10 out of 10 in focus, in bright outdoor light. Of course if the background or foreground is busy, or has more contrast than the object...and these subject matter are very close to the target object in the field of view...yes the 6D can lose lock that way as well. So in less than ideal light with all points active, while servo tracking...I agree the hit rate drops to anywhere from 6 to a terrible 3 out of 10...especially for a fairly fast moving subject. But then that's why you pay the extra $1100+/- for the 5D3...Again the two cameras weren't meant to compete with each other directly. It's just that in some aspects they do.