A great set of answers from Rachel to frequently asked questions about CSS grid. She addresses the evergreen question of when to use flexbox and when to use grid:
I tend to use Flexbox for components where I want the natural size of items to strongly control their layout, essentially pushing the other items around.
A sign that perhaps Flexbox isn’t the layout method I should choose is when I start adding percentage widths to flex items and setting
flex-growto 0. The reason to add percentage widths to flex items is often because I’m trying to line them up in two dimensions (lining things up in two dimensions is exactly what Grid is for).