I tend to like using Grid and Flex rather than frameworks. I used to use Zurb Foundation, but found myself overriding the elements and css classes (or at least portions of them) more often than I felt was reasonable. I do use Grid and Flex within the same project often as they have different strengths for doing different things.
Over time, I felt that using the built-in CSS3 tools just gave me more freedom. I didn't really have to remember all of the class names for foundation (is it small-column 3, small-3 column, or small-3 cell....) and could focus more on the design my wife has given me (she's the UI/UX designer for our consultancy) than bouncing back and forth between my project and the docs for a given framework.
That is just me though. Personally, the less I can change gears, the better. The time savings of writing 20-30 lines fewer of CSS and HTML usually gets offset by me being "thrown out of a groove" when laying things out. I totally agree though, each have their strengths. For me, I'd rather use the languages built-in, standard lib stuff than reach for an external framework. Usually because if I can build something in straight-up CSS, or Vanilla JS, or Python's standard library, I can better understand what's happening under the hood if and when I need a dependency. Just works better for how I personally approach problem solving.
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