Both Less and Sass implement features that I consider anti-patterns: nesting, @extend, etc. Sure, you can ignore them, but there's a lot of code in the tool to support that. The code needs maintenance and puts the barrier of entry higher for those wanting to contribute to it. Also, these are first-class features of the tool, they can't be disabled explicitly. So you'd need code reviews and other artifacts to make sure no programmer/designer ever tries to use them. Finally, why would you choose a tool that does 20x what you need, if there's an alternative that does just what you need? (As stated in other comments, I don't mean my solution is what will replace all preprocessors. PostCSS looks like a more modular approach and it allows you to effectively cherry-pick the features you need by means of plugins.)
Also, easy != simple. The fact that a tool is easy to install (after having installed another mega-dependency) shouldn't count as an advantage, in my opinion.
Nesting is the only reason I wish to use any of these preprocessors to be honest. I don't understand how people write plain css when things like less/sass exist. Writing and maintaining css selectors is a nightmare, but it's so simple and intuitive to just nest things inside parents.
I was going to say. How the heck is nesting an anti-pattern? It is SO useful for CSS. It should have been standard years ago. Variables are helpful too.
Also, easy != simple. The fact that a tool is easy to install (after having installed another mega-dependency) shouldn't count as an advantage, in my opinion.