Tracking reliability: Eye tracking accuracy may degrade a little in changing light as your eyes adjust to the new conditions. While this shouldn't necessarily affect your application design, as the accuracy should be within the 2° limitation, it may be necessary for the user to calibrate again.
Ragged eye-gaze movements: Our eyes perform rapid movements from fixation to fixation. If you look at scan paths of recorded eye movements, you can see that they look ragged. Your eyes move quickly and in spontaneous jumps in comparison to head-gaze or hand motions.
Focusing Operations in Z direction: while our head may stay in the same general direction, our eyes have the ability to perform focusing operations for different levels of depth