Both create more problems than they solve, both essentially break the advantages of css. What bothers me about css-in-js, it does not have a single advantage over vanilla css. Every challenge it supposedly solves is a real world non-issue if you know how css works. Local styles, theming, dynamic styles, safety, maintainability, performance, you can have all of this in vanilla css without the issues associated with inline styles and the js-in-css abomination. Just learn the damn thing. js-in-css is a lazy workaround for software developers who want to do things the way they learned, not the way it was meant to be done.