UnCSS removes unused CSS from your stylesheets.
PurifyCSS does not modify the CSS source files.
I use PurifyCSS and I totally recommend it.
I hope that it will simplify everything for you, everyone has a way to do what is needed. If I were you, I would test them and see which one is more useful to you.
EDIT:
PurifyCSS also takes care of JS files that CSS classes and identifiers can add.