You can compile React to Webcomponents with community tooling, the core framework just doesn't support them:
https://github.com/adobe/react-webcomponent
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).
