I've NEVER seen any advantages to any CSS pre-processor that couldn't be equally accomplished by simply knowing how to use CSS properly, easing up on the mental midgetry of throwing classes at EVERYTHING, and bothering to leverage your semantics and CSS selectors.
Which is how I end up with 12 to 16k of markup where people using CSS preprocessors, OOCSS methodology, and mind-numbingly halfwit bull like front-end frameworks blow 60 to 100k!
MAYBE if people stopped using hundreds of K of CSS spanning a half dozen or more files to do the job of 32k or less in one file, they wouldn't be diving for nonsensical pointless garbage like SCSS.
All I've ever seen it done is further enhance the ineptitude and ignorance of those who use it. I've NEVER seen ANYTHING done with them that couldn't have been done more efficiently and effectively without them, unless it was something that has zero damned business on a website in the first place!
Oh, and Tommy Hodgins that first claim under "advantages" in your post? I've never seen that one be true... at least not by the time the preprocessor finishes off vomiting up whatever mess it has the unmitigated gall to call CSS.
Don't even get me STARTED about the derpitude of doing CSS from JavaScript, which is why everything wrong with SCSS gets multiplied tenfold in PostCSS. Controlling appearance from JavaScript for anything more than an animation enhancement (which even then should only be a class swap to trigger CSS3 doing the heavy liftting or height as a bugfix) -- EPIC /FAIL/. Do not pass go, do not collect $100.
Even server-side, it's dubios -- the tutorials about PostCSS all showing examples that would be LESS code without it! The "custom selectors plugin" being the poster child for how PostCSS is in the same class of stupid as w3.css! What the blazes type of brain damage does it take to see an advantage in this?!?
Much like the idiocy of HTML/CSS frameworks, I cannot fathom how or why anyone would choose to use these systems apart from knowing so little HTML or CSS they are unqualified to be using either in the first damned place!