I found this website and I would like to share some of my experiences:
First, the best flicker-free device I have worked with so far is my Ipad Air (1). I don't have any other devices from apple.
Not why is that?
When you want to repair an Iphone (or Ipad) backlight, you won't get to buy any replacement LED‘s. Instead you will only find a very flat panel, which is an EL-Panel and shines bright by setting it under relatively high voltage (200-300V) and an alternating current with about 300 Hz. The current is very low though. Now 300 Hz doesn’t seem very high, but I cannot register any flicker. Also the brightness can be reduced by dropping the voltage, so it doesn’t use any PWM.
Ipad Pro and a lot of „old“ non-OLED (so non Samsung) smartphones still use a similar flicker-free backlight.
I am currently writing this text on an Ipad serving as a clone-display for my old Windows laptop (it‘s a bit small) and I have connected them with duet display. I feel no eyestrain at all.
Unfortunately, there are no laptops on the market, which use this sort of backlight. You could build one yourself by putting an EL-Panel behind your laptop screen. It would be a bit complicated though, I think.
What also helps me with PWM-backlights is putting a flicker-free CCFL-lamp behind the display and covering the lamp with a deep green color-filter.