The worst LED lights are 60Hz, very short duty cycle, very quick flash, with no diffusion. They're seemingly-infinitely small pinpricks of pure chroma flashing at an unpleasant speed. The worst are the blues.
I can't focus properly on bright blue things. There's a football stadium not far from where I live and its name alternates between red and blue in big text.
I know what it says but I cannot read it at night when it is blue.
No one can, for a bunch of reasons. The eye is mostly corrected for red-green wavelengths; the eye is a lot less sensitive to blue light; there are far fewer receptors for blue light, which reduces spatial resolution.
Monochromatic blue displays are an extreme UI antipattern and should not be used for anything. If combined with high luminosity, they become essentially unreadable unless close up. E.g. ultra-blue seven segment LED displays. VFDs can suffer from this, but this is also the reason why many of them use green-blue phosphors, and not blue phosphor (which would be possible).
Best way I can describe it is that my eyes can't focus on them and are constantly trying to.