Thank you for your questions, I’ll try to address them one by one.
All Tobii eye trackers use near-infrared light in illuminators and sensors, allowing “accurate, continuous tracking regardless of surrounding light conditions” (copied from http://www.tobii.com/en/about/what-is-eye-tracking/)
As I said in another thread, the accuracy/precision/robustness of the EyeX Controller is being continuously improved, so there is no point in comparing it with other eye trackers right now.
Regarding head tracking, the EyeX Controller compensates for movements in all axes, but with some delay in the z-axis (since it is a mono-system with only one sensor, it requires the head to be at the same distance for a number of frames before it can compensate).
So, using the eye tracker as a head tracker is probably not as accurate as dedicated head tracking system at the moment, but probably good enough for most applications. In the next version of the EyeX SDK for Unity we will include a code sample demonstrating how to use eye position data, to implement head-tracking-ish functionality in a game. We hope it will be useful for developers who want to experiment with this data stream and create innovative games based on the eye-gaze point on screen as well as the eye/head position.