I get the error:

Input.error ((execjs):58303:16): CssSyntaxError: /stylesheets/screen.css:5:1: Unknown word (ExecJS::RuntimeError)
  3 | @import 'variables';
  4 | @import 'icon-font';
> 5 | // uncomment to switch to RTL format
    | ^
  6 | // @import 'rtl';
  7 |

This is because Slate is attempting to use the sassc gem to compile the scss stylesheets. You need to make sure you are using the sass gem. You can accomplish this by making sure the sass gem is in your Gemfile and that middleman-sprockets is on ~>3, and not using ~>4.

I get an error that says WARNING: V8 isolate was forked, it can not be disposed and memory will not be reclaimed till the Ruby process exits. and attempting to build hangs / fails.

This generally means you're attempting to use mini_racer as was recommended in older versions of Slate. This gem has issues working properly with Slate, and it is recommended that you just install Node.js runtime and remove mini_racer from your Gemfile.