Inconsistencies in the source file do not affect production worthiness in the moment, but they do affect maintainability down the line. If for instance your team wants to move to a new css preprocessor that's the next big thing in 10 years, a consistent style means you can programatically convert to the new syntax. Or maybe there's a tool that can help you write better css and detect where rules are unused -variable syntax will be sand in the gears for something like that. Mishmoshed syntax everywhere is a nightmare for programatic accessibility, though is excellent for simply trying things out.

I love stylus, don't get me wrong. I use it for all my projects - even in production. But if I were the head engineer of say, google.com, I wouldn't allow it in production for a codebase that large due to the potential for problems down the line.