I have recently (2 weeks) started to use Understrap as starter theme kit for Wordpress. There is a lot of pre-developed functionality to make it easier to override or add additional features to a Wordpress website. The kit uses Underscores starter theme and Bootstrap 4 which have both matured in the past months, and uses SASS, npm and Gulp to pre-process all the files and make the necessary performance improvements which are very important to keep a website working smooth.
So far I have modified slightly some code especially the gulpfile as I was not getting the wanted distribution code so I had to add some extra code. I hope to add and change some more stuff soon.