Thanks for your reply! Completely agree with what you're saying, but I think that even though it might not be 100% valid CSS that the Sass compiles to, if it doesn't break down it means there is at least a style sheet that works. CSS errors could be there even without Sass, I don't think Sass is the problem there :).
A bigger question for me is though: how do I actually get to automating all this? Do I install tools on the server on which the projects run? Or do I use another server that builds and checks and validates everything and then pushes the valid built package to the server that serves the application (website)?