Assuming you mean actual "custom properties" from CSS3, the lack of support in older iOS and Android devices and complete lack of IE support IMHO still makes them non-deployable on WEBSITES.
If you mean using a CSS preprocessor I consider that a pointless extra step that is just making you work harder, not smarter.
Though in EITHER case -- much like the 'mixin' garbage' -- if you "need" them or they provide ANY benefit, it is likely you've got bloated overthought CSS, possibly with bloated overthought HTML to match. If you are leveraging selectors properly, you just don't need it.
But I say that from the perspective of someone who watches the 60-100k of markup, 250-500k of CSS and 500-2048k of JavaScript people vomit up on websites in complete wonder how anyone could be THAT STUPID since in most cases there is likely I would be using 8-24k of markup, 24-48k of CSS, and 0-48k of JavaScript to do THE EXACT SAME JOB!
ALL because unlike most people I've embraced semantic markup, aren't dumb enough to throw five or six presentational classes at every Joe-blasted element, and don't buy the idiotic LIE that complex selectors have performance issues compared to the alternative -- bloated markup and specificity hell -- even if both Google PageSpeed and Yslow are making that claim.
In the traditional "If you don't know what's wrong with this:"
class="theme-handbook-template-default single single-theme-handbook postid-11273 admin-bar no-customize-support single-handbook"
"Back the **** away from the keyboard and go take up something a bit less detail oriented like macramé" kind of way.
Realistically there is NO excuse for the majority of websites to 'need' more than around 48k of CSS to do what they are currently doing apart from utter and complete developer incompetence and/or ignorance. JUST like there is no reason for anyone's markup to have a CtCR (code to content ratio) of more than 3:1.
... and again because of that if you have enough CSS that something like "variables" has you thinking you 'need' it or is providing legitimate improvements, you likely have too much CSS; with too much HTML to match.