The problem with HSL gradients is that you now have a completely different color, and the whole thing looks like nothing found in nature. Unless the rainbow is

The problem with HSL gradients is that you now have a completely different color, and the whole thing looks like nothing found in nature. Unless the rainbow is an intentional design element, it just doesn't quite look right. It's useful, but not a substitute for really carefully picking your colors so it all works. One of the harder things in creative projects is when you have two individual elements you want to use, but you don't like what happens if you add both.