I can't speak from experience with the 6D, but based on the AF system, it's not going to be optimal for BIF. A higher density of AF points, along with an expansion mode where the nearest point is used if the subject drifts off the selected point, make tracking a BIF much easier.
Certainly, a 6D can be used for BIF. People shot BIF with manually-wound film cameras before AF lenses existed... But with a 5DIII, the camera will do a fair bit of the work for you, whereas with the 6D, it'll be mostly down to your skill.