Webp is more efficient than jpeg (almost twice less filesize for equal visual quality). Webp compression makes image look smoother, which is usually good as it reduces graininess. While jpeg introduces ugly artifacts near borders of colored areas. To avoid that you would need to use jpeg with better quality (larger files).
Bottomline: no, webp is not overrated (and webm for that matter), it's just the best. Especially for 3DCG renders. Only small pictures like UI buttons don't compress well into webp; then usual png is OK for them.
You get grainy renders when you "undercook" rendering of a 3D scene into 2D picture. Imagine rays of light that bounce from objects and create colored dots in the resulting picture. When there are very many rays, they form a smooth picture as the result. But if you try to work faster and use lower amounts of rays, there are separate dots in the image.
It has nothing to do with image format. The developer just wanted to make more pictures in shorter time, at the expense of quality.