The problem is if their PWM is too slow, to the point where we actually see the flicker directly, or just barely indirectly (e.g. by seeing a "slideshow effect"

The problem is if their PWM is too slow, to the point where we actually see the flicker directly, or just barely indirectly (e.g. by seeing a "slideshow effect" when you move a finger back and front against the lit-up display). Concensus among opticians and ophthalmologists is however still that the main cause for eye strain is focusing your eyes at a short distance for an extended period of time.