It makes my eyes physically hurt, to where I can't think or focus on anything. It feels as if my eye is up next to the screen of a window and just touching it.
Not all PWM has issues: I have an LG Plasma TV and it has PWM, but the way the plasma glows compared to the rigid on/off PWM cycling on modern iPhones means the Plasma display causes no issues. Same with Samsung Note9, which uses OLED with a soft PWM ... no issues
PWM is awful. I can tell within seconds of seeing a screen if it has PWM and usually I start to get eye issues within a few minutes.