There is also a section in the readme explaining the differences with the most used libraries: https://github.com/FullHuman/purgecss#differences-with

Uncss works the following way: The HTML files are loaded by jsdom and JavaScript is executed. All the stylesheets are parsed by PostCSS. document.querySelector filters out selectors that are not found in the HTML files. The remaining rules are converted back to CSS

Purgecss will not emulate the dom and execute javascript. Instead, it will read your files, determined the selectors in it and then remove the ones that are not used. A much simpler process that will translate in better performance.