Web components aren't really there yet. They will be two or three years from now. Some time between now and then, I expect React will gain the ability to compile down to them, which shouldn't be too hard since web components are pretty much what happens when the React model gets pulled into core.
By "aren't really there yet", what do you mean? If you mean in a sense of public adoption and awareness, totally agree.
If you mean that they don't work properly, heartily disagree. They function just as well as custom components in any framework, without the problem of being vendor-locked.
You may not be able to dig in to the internals of the component as well as you would a custom build one in your framework-of-choice, but that's largely the same as using any pre-built UI component. You get access to whatever API the author decides to surface for interacting with it.
A properly built Webcomponent is generally indistinguishable from consuming any other pre-built UI component in any other framework (Ionic built a multi-million dollar business of off this alone, and a purpose-built framework for it).
