wordpress vs frameworks

I've been seeing these framework sites grow a lot this year. Every now and then a new one pops out.

• bootstrap
• semantic ui
• materializecss

Has anyone experience with these or similar tools? How do they compare to Wordpress?
These frameworks look very light weight compared to all the baggage that comes with WP.
I run a handful of sites, mainly for small-business clients, that look like they can benefit as they're all static and can be replaced by simple html/css/js files rather than a big CMS.