Hi,
For classic graphics the shader has made (almost) every single unit portrait look darker, as if it is standing in the shade. Many of the visual effects are much, much darker and less noticeable like the healing salve twinkle. One of the main reasons WC3 is popular is because the colors popped out (both for playability reasons and for visual appeal).
I get that it probably works for the reforged graphics, but most players still use the classic ones.