Boost is the only core theme using SCSS and Mustache (and Bootstrap4) - Clean and More use LESS and Bootstrap2.
There are a couple of Boost based themes in the plugins database (Photo and Waxed are good starting points for further development, while Fordson is a fully featured theme). There are also some notes around a pico presentation I did at the UK Moot earlier this month at https://www.slideshare.net/RichardOelmann/moodle-moot-ieuk-2017-creating-a-child-theme-for-boost
There are also a couple of themes that use Bas Brand's theme_bootstrap as a base/parent using Bootstrap3.
Boost is the way forward but I'm personally not convinced its ready for full production yet (that's not a comment on the theme itself, but on the fact that the upstream BS4 is still in alpha), but if you are happy to take on that overhead, then its certainly the direction HQ are heading. That said, I'm the author of Waxed and am working on few other Boost based themes as personal learning projects for development, so quite happy to join this journey
Richard