One real problem in using the hue component of HSL is that it wraps around perceptually by design. There's no "greatest" or "least" hue. And the ordering isn't exactly intuitive because we don't perceive colors based on the order of their respective wavelengths on the EM spectrum. Varying the saturation or luminance component while keeping the other two constant makes it much easier to perceive the relationship of two colors at a glance. Or just take a gradient between two appropriately contrasting colors which is exactly what viridis is about.
