Its inconsistent in varying conditions, all cameras are, more or less. Of the cameras i tested Pentax has this, but on the other hand Pentax accuracy and speed in af-s centerpoint under good conditions on static targets is exellent. In the same level as other brands. Switch to af-c and trying to track small children with odd coulored clothes, well... Thats another story.
I never use anything other than af-c in tracking (3D track) with my nikon. My shooting is like that, hence my choice. However, just about everything else is better with Pentax.... Hence... (weird word) my prescence here