The designer tool allows you to edit Sass, Less and PostCSS in the browser, which can be saved to the main file.
SC5 is built on, and extends the functionality of KSS. It is compatible with existing KSS styleguides.
There are some existing themes available (such as material design). Or you can use the PSG-theme generator.
Since it is a PostCSS plugin, it's very fast to set up for those already using PostCSS and integrates perfectly with the PostCSS workflow.
Getting started with KSS can feel complex compared to other living styleguides, which is even admitted in the KSS documentation.
Hologram has a variety of plugins and extensions to choose from, and the ability to create custom renderers.
Hologram supports Markdown and JavaScript, as well as popular preprocessors (Less, Sass, and Stylus).
Currently Hologram has limited error reporting, sometime giving unhelpful messages or failing silently.