Einks usually degrade at fast refresh rates (or too many or too little reflashes), but that might be just the cheap ones (otherwise this product wouldn’t last more than a few years, which hopefully ofc isn’t the case).

But the “brightness” limitation of colour einks still bothers me a lot, the contrast on/of white backgrounds is just too good on monochrome versions (eg for books I don’t want colour einks), and it’s not just something I notice in direct comparison, it’s a constant feature/luxury.