> have the advantage of perfectly adapting their brightness to their surroundings.
The surroundings might not be lit enough for the health of eyes. This is the usual case and one of the prime issues why dead tree books and e-paper tablets lead to weak eyes.
There's evidence that lack of exposure to sunlight increases risk of myopia. But emissive displays are typically dimmer than sunlit objects, even at full brightness. If insufficient brightness is a concern, you're better off using reflective displays outdoors.
People often read at night before going to sleep. If reading in the Sun was a requirement, one could do that with physical books too. However, most reading happens indoors and for that an iridescent panel screen such as that on an iPad with full gamut of colors is suited better.
