I have two Dell 2407 displays at home - CCFL and no detectable PWM, which have been great for me. I thought however it was time to get something bigger and better for gaming & simming. So, after much research I got an Acer XB271HU (27" ~IPS flicker free 144Hz G-sync) and it looked amazing, but sadly gave me rotten headaches. So I tried a VA 34" Samsung screen out and it too gave me headaches, as did the Dell 34" 21:9 display. Right now I'm pretty much resigned to having to stick to my tried and true CCFL dells.
Like others here I can watch plasma and ccfl screens A-OK, but pretty much anything else is no good. Frankly I cannot work out just what it is about these LED displays that don't agree with me. I must admit I live in fear of my big 54" Plasma TV dying on me!
Puzzlingly though, my late '13 15" Macbook is A-OK as are my iPad Air 2 and iPhone 6S, where pretty much everything else including earlier i-devices are problematic.