Have you tried adjusting the brightness of the screen on your non-eInk devices? I suspect that the difference between the screen's brightness and the ambient light is a cause of eye strain. If this is the case, eInk readers would have at least two advantages: with the light turned off, the screen's brightness reflects the ambient light (i.e. there is essentially no difference). Even when the light is enabled, ereaders seem to have significantly lower and more fine grained brightness settings. That makes it easier to reduce the difference between the screen's brightness and ambient light. (Heck, some of the Android ereader software that I've used will replace whites with greys in order to reduce the apparent brightness by reducing the contrast. Alas, that doesn't do much about the back light bleeding through.)