Great and extensive article, but the thing missing are the two major drawbacks of WebP:

• Quality. I’ve done a lot of experimentation and come to the conclusion that WebP achieves its reduced file size by washing away details, resulting in quite blurred images. It may not show in your best case scenario, but try it on some larger images and it becomes very obvious.
• Side effects. Such as users wanting to download an image and not having anything to open it with, which is why Facebook stopped their WebP experiment.