I’d argue that using CSS-in-JS may be good for developers who are more familiar with Javascript and are new to CSS. But using modular SCSS lets you keep your CSS unique to a component. It also gives you the opportunity to reuse styling code and take advantage of specificity and overriding, where it makes sense to.

HTML and Javascript work together to create your content and event handling, so it makes sense to see them together. I’d rather not have my CSS in there, too. Too much clutter.