BS4 includes Flexbox and it has been designed to be much easier to customize with SASS. They have also added a bunch of utilities for spacing, colors, etc which makes customization at the CSS level easier as well. For a practical example, I only had to write a few lines of SASS to give some components (e.g. modal) a material design look. I think your criticism is valid for BS3 but not BS4 unless you are building a website that is drastically different than 95% of websites.