JSX is so good because it's an effective language extension, but CSS-in-JS still feels like a dirty mess, everything just becomes strings in objects, it just takes the elegance of the CSS syntax and throws it out the window.
Unfortunatelly this is the state of software these days - half-baked libraries + incremental improvements. This doesn't work in most cases. I wouldn't call it mistake though, open sourcing anything is a good thing.
The right solution is a library like this with proper cross-platform abstraction. Then you can simply compile css-in-js to stylesheets to have caching, therefore fully utilizing platforms capabilities.
Who cares whether css is written in css files or js files or whatever. Abstraction + solid tooling (compilation) + ide support is the way.
Like, look at this:
Hmmmm