Hi there,
our intentions is to switch the themes use of Unsemantic over to pure Flexbox CSS. We’re already using this in the Mobile Header for example and with some CSS Grid in the Woocommerce module.
This means there will be no CSS library/framework dependencies in the future, albeit existing sites will have a choice of keeping unsemantic if they have been using it in their own HTML/CSS
Personally, if i am working with HTML – i use Flexbox and CSS Grid. I cannot see there will be that many pre-defined CSS rules in the future theme for you to take advantage of. And as flex is so lightweight to begin with i will continue to write my own custom CSS for this.