Yes, Tobii REX follows the same standards as the PCEye Go and is safe for longtime exposure like the FAQ says. (The standard is called IEC/EN62471, Photobiological Safety of Lamps and Lamp Systems if you are interested).
That said, a large number of factors can affect eye fatigue. Both the obvious ones like humidity, surrounding lights or blinking regularly and the more tricky ones like how the gaze interaction is designed. Erland posted a good summary of eye gaze interaction earlier this week, have a look if you haven’t seen it already.