I don't think e-Ink technology is really suited to smartphones. It was designed to display black text on a white background. Anything more is pushing the boundaries. Unbox Therapy did a review of one not too long ago, the Kingrow K1, and it was pretty abysmal:
https://www.youtube.com/watch?v=vS93u75NnPo
I like the idea of a minimalist phone but they don't need to resort to e-Ink for that. An OLED device with system wide dark mode and software optimizations could achieve the same effect.