I read everywhere that both are complimentary. I am a complete amateur in web dev but the only place where I can imagine flexbox in an otherwise grid display is when divs are stacked and we do not know how many divs there will be.
If grid has a way to address these unpredictable divs (not knowing how many there will be) then my last use case is closed.
I'm definitely more of a backend developer myself, but of course I have to do the frontend work on my own projects.
For my use case, I'm using CSS Grid on this project (https://www.contabulo.com) to manage the layout of 'cards' on a 'board', where the number of cards is variable. I was originally using flexbox, which proved to not be.. flexible enough to manage it properly (and there were some annoying behavioral differences between browsers).
I was worried I was going to have to resort to some Javascript-hackery when I happened upon CSS-grid being mentioned in an HN post a while back. Luckily, support for grid was added to all of the major browsers not long after I discovered it :)
If grid has a way to address these unpredictable divs (not knowing how many there will be) then my last use case is closed.