A monolithic style sheet is difficult to reason about, which is challenging as CSS files tend to attract bloat. Removing unused styles can help make the situation more manageable.
Part of the returned results will likely include Remove unused CSS rules. If this is missing, it means you don’t have any unused CSS rules. Congratulations!
I initially preferred UnCSS and the command line convenience. However, since drafting this article I’ve tried this out on a few more sites and had less promising results. Most notably, I have several sites built a few years ago that made use of a framework that implements persistent comments using the /*!*...*/ notation. This doesn’t play nicely with PostCSS and thus UnCSS. I haven’t dug into the issue further, potentially a newer PostCSS script is more forgiving of this type of comment, but for now it’s another barrier stopping UnCSS becoming a more common part of my workflow.