- Simple horizontal, vertical, or radial gradients can rarely capture the "organic" look that most designers want (the gradients of real materials and scenes are highly uneven so perfect symmetry has a distinctly artificial appearance).

- I'm not sure if it's actually an aspect of the spec, but all implementations I've seen interpolate the colors in RGB or an RGB-like colorspace. This can have the effect of creating dramatic jumps or ridges. With a lot of trial and error (or color math) you can overcome this by using custom stop values, but it's much simpler and more in-line with a designer's toolset to be using colorspaces whose linear interpolations are more perceptually uniform.

Of course there's also the "standard drawback" of modern webdev too: it's often easier to just throw assets at the problem. :/

(Thoughts from a design-adjacent dev.)