Alright, I just spent the entire day on greyscale yesterday to check.
I think that I suffered more eyestrain than usual, mostly because I could not tell icons and other UI elements apart. Yet I don't have this issue on the E-Ink device, interestingly, even though I use many of the same apps e.g. Telegram.
I would say that the difference is due to the reflective vs radiative screen, not the greyscale palette.