Bootstrap 4 + Flex, how to use it better?

Question

HTML

I recently started learning bootstrap 4 and its combination with flex.
Twitterstrap has built-in classes that add flex to items (d-flex, align-items-... etc.). The question is whether it is correct (correct, justified) to use these classes (directly adding them as a class) or is it better (more profitable) to write in your styles through mixins?
It is the problems and advantages of both methods that are of interest.
P.s. The question is more for experienced layouts and users of Bootstrap.

Answer 1

Just an opinion: light up the nets fire up bootstrap, foundation, flexgrid, etc. gridsWrite your own bike, compare everything with everyone else, take the one that suits a specific project and forget about the bootstrap.