It is wildly highlighted that Flexbox is for 1-D and Grid for 2-D but I have not found a clear explanation why Grid could not be used for 1-D and replace Flexbox. The closest I came to is
But you could also argue that purely 1D layout like this is more powerful in Flexbox, because Flexbox allows us to move those elements around easier (e.g. move them all to one side or another, change their order, space them out evenly, etc).
I use Grid and Flexbox for basic layout: a general placement of the boxes on the page and some dynamic ones, usually stacked. The esthetical ones (toasters, modals, ...) are managed through a framework. I have not yet found a case where Grid could not replace Flexbox out of the box (that is without advanced CSS gymnastics or a lot of code).
To take the example of the quote above, all the "moves" mentioned are directly available in Grid, usually with the same semantics as in Flexbox.
What are the fundamental areas covered by Flexbox which are difficult or impossible to manage with Grid?
EDIT: the browser support is not important (I only use evergreen browsers and can switch if needed)