For me no framework is the best framework. Usually what you end up using mostly from these is the grid system. With flex and grid in CSS I don't feel the need for a grid system anymore. Usually the components being shipped are not being used 1:1 so you end up customizing/overriding the styles where you can waste just as much time as defining your own styles. In most cases and on big scale projects it's just simply not worth to go the CSS FW route. In case you need a fast UI for some admin interface, than go for it.
For me no framework is the best framework. Usually what you end up using mostly from these is the grid system. With flex and grid in CSS I don't feel the need for a grid system anymore. Usually the components being shipped are not being used 1:1 so you end up customizing/overriding the styles where you can waste just as much time as defining your own styles. In most cases and on big scale projects it's just simply not worth to go the CSS FW route. In case you need a fast UI for some admin interface, than go for it.
Thanks for your comment.