This thread doesn't make sense to me.
Let's break it down..
On an e-ink e-reader, the screen only refreshes when the page is turned. It is not continuously refreshing. This is not a monitor we are talking about here.
So, unless the issue is something physical like the resolution of the panel or the coating reflectivity or the contrast ratio between black and white on the panel, what else is there? I can think of two things.
Backlight, I saw a good Kobo the other day outside in the sun without the backlight on, and it looked very legible and clear. But if you are using backlight, then sure, that might have PWM. I don't think the newest Kindles let you turn off the backlight, but the Kobo does, though it probably does have PWM (who knows?).
Font rendering.. being able to adjust anti-aliasing and the bytecode interpreter is a good thing to have, and I can't say who does it better or worse but if you are careful with the font you use, no matter the settings, you should be able to find a font that works for you. Beyond the technology of the original design (e.g. the original Microsoft core web fonts were designed without AA being expected to cover up things and they are very well hinted for low resolution displays), the shape and weight of the letters are a very subjective personal preference. For example, I dislike Bookerly, Amazon's new Kindle font. I think it sucks. Here is one font I played with recently that I liked, IM Fell DW Pica, sideload/install it into your device's fonts folder and give it a try: https://fonts.google.com/download?family=IM%20Fell%20DW%20Pica