> They are supremely useful. Ultimately, it's knowing how to use them to their maximum potential.
Supremely useful for a typical full stack developer -- yes. Supremely useful for a dev team with members who specialize in web design (plain HTML, CSS, etc) -- not necessarily.
When your source code mixes a bunch of different web technologies it limits the types of developers who can effectively work on it.
Many commenters here regard React and Vue as "easy", but for many designers it's not easy. There are plenty of talented web designers who are just not interested or not capable of working efficiently with React/Vue.
Our company built an in-house JS component framework that is heavily inspired by React and Vue but that doesn't suffer from the intertwining of disparate web technologies. HTML, CSS and JavaScript stay separate from one another. As a result, our designers can independently iterate on UI without needing to touch a bit of JavaScript or quasi-JavaScript constructs.