I don't find plain CSS harder to write. It's a bit more verbose and unintuitive to think about sometimes with long-ass property names, too many ways to do the same thing, a bit too low level etc.

Tailwind simplifies them and make those things more readable as well as visible. Combine that with postcss, you get ton of functionality via plugins.

You can use tailwind in any project. The end result is pretty minimal after purging unlike bootstrap or traditional CSS frameworks. Think of tailwind as high level styling framework that simplifies many common scenarios you tackle with writing few lines of CSS.