CSS grid is strict rows+columns, flexbox is 1-dimensional and allows for controlled wrapping to a second/third/etc row when the row (or column, depending on orientation) is filled.
They have different use cases, it's just that (at least with regards to (a)) everyone's switching because they wanted something like grid in the first place and only had/knew about flexbox. It got you halfway there (we've used multiple flexbox rows that don't wrap to simulate grid, for example), and now they're just going all the way.
I think the parent new all this and was saying that when using flexbox with grid, each for their intended purpos, there are bugs in various browsers for them.
They have different use cases, it's just that (at least with regards to (a)) everyone's switching because they wanted something like grid in the first place and only had/knew about flexbox. It got you halfway there (we've used multiple flexbox rows that don't wrap to simulate grid, for example), and now they're just going all the way.