It's just my opinion of course :) but I think css-in-js is a step backwards in the majority of cases. I do like CSS modules in terms of scoping / modularity, but I don't want to replace CSS with (what I consider to be) really gross JS implementations.

I don't want to discourage people trying new things, but I also don't want people to waste thousands of collective hours dealing with crazy configuration.

I think toying around with those things are fun for people who get paid by startups etc, but for those of us who simply want to get things done, the ecosystem is not enjoyable.