In my experience and for my eyes, yes the facts are simple: CCFL is always good, LED is always bad.
For the past 20 years (working a minimum of 8 hours every day, stylesheets, coding, photo, video):
CCFL: desktop, laptop, Windows XP to W10, Linux Mint or Ubuntu => no problem, no strain no headache. And my old CCFL screens do flicker a lot and it's not a problem (though CCFL flickering is "softer" than LED flickering).
LED: smartphone, external monitor, laptop, TV, same operating systems (Windows, Linux), Low Blue Light, Flicker Free, Asus splendid blue light filter, Android filter, Windows night setting, Iristech software, all kinds of settings, low saturation, low brightness, zero contrast, ...
=> immediate pain, headaches, brain pressure, heart beat too high, etc...
In fact, contrast, brigthness, ambient light don't matter with CCFL. Even with not-so-good settings and a bad environment, CCFL do not hurt. While LED, even in a perfect environment with fine tuned settings always hurt.
So I still don't know what it is but I can draw a practical conclusion from that: LED is bad for me, no matter the software or hardware around it.
There must be something else "in" LED light, something beyond blue light.