It seems like Web Components are ideal for libraries shipping pre-built components, which is probably why Mux finds it compelling. From Mux's point of view, they want the highest level of compatibility with the least amount of framework lock-in. For example, they don't want to have to ship a library for React and another one for Vue and another one for bare bones JS/HTML.
In terms of building a web app where you control the environment end-to-end, I don't think there's any inherent upside to using Web Components over React.
In terms of building a web app where you control the environment end-to-end, I don't think there's any inherent upside to using Web Components over React.