I don't think the eink part of it has issues with yields since it's essentially just conductive ink and a suspension of particles, but the fancy transparent transistors they sometimes use certainly do. The real issue with eink is the economies of scale will probably never kick in. Most people would rather have a color screen, or a screen that exceeds 10fps, or something that doesn't have artifacts like an etch a sketch, etc. There's only 1 oem for the entire eink panel market (over 6") last I checked.