Well that that point it's slower to paint with CSS-in-JS because the browser won't have the CSS until the very last thing (after asset load, after initial paint, after content load, then after script execution).

Now you could extract the CSS with a build step... but at that point you've admitted the solution is poor (and/or you're not good at CSS) and are just mitigating it by producing real CSS by automation.

That's fine, but it's no replacement for general good practice or domain knowledge.