We wanted to follow the PageSpeed Insights CSS recommendations and separate critical and deferred CSS. To that end we read lots of posts and recommendations, such as Optimize CSS Delivery and Eliminate render blocking css.
We also started separating the CSS physically, but quickly came to the conclusion that if we do it in a way that all pages show correct above-the fold content, about 70% of the CSS would need to go inline, leaving only about 30% deferred.
Having a large portion of the CSS inline doesn't seem to make much sense, so we wonder if we can ignore the PageSpeed Insights suggestion without too much ranking penalty. At present, how much SEO difference does fixing this issue really make and is it worth the effort for a site with several different page layouts (different categories each have their own layout)?
You start inserting above the fold CSS when it makes a measurable difference. Is inserting the CSS making the page display faster than round-tripping a CSS file? You might look into why 70% of your CSS is being applied to the top of your page.
Then again, a lot of recommendations by Google, and others, are for large sites with large pages and megabyte files; something to keep in mind. The documentation won't always state exceptions.
The best thing to follow is the network and timing tabs in your browser's developer tools.