Accidentally deleted what I wrote, but in essence you often use hue shifting in colour ramps to create a greater level of contrast, avoid a dull colour ramp and to try and replicate that kinda Impressionist idea that highlights are complementary to shadows. Like for tree foliage, you might use warm greens in the highlight because the light from the sun is yellow, and blue-greens (or even purples, to push it to the extreme) in the shadows to replicate scattered blue light from the sky/give the yellow highlights higher contrast.

I think I'm right in saying that using drastically different hues next to each other in a ramp isn't considered hue-shifting, but rather an experimental thing or out of necessity from using a set palette (also to neutralise etc., but I don't have a great understanding of that)

Like in this piece here, where the cheekbones are orange next to green skin.

I dunno, the green on the skin just make's the skin look unnatural.

I feel it has to do with the hardness/texture of the object in question;

Less hue shifting for softer surfaces and more hue shifting for harder surfaces.

I'm speaking purely out of realism, there's no wrong way to make art!

in order to mimic, you have to understand it.. Studying light will always help with your art. Don't hue shift simply to hue shift otherwise you'll just create a habit and never question what you're doing. Good artists always question their work and try to see all the angles involved with it and make corrections where necessary.