Until you actually try them yourself in various browsers (and versions) with dynamically sized content of the elements inside those layouts and see that it is still buggy and not that much has changed and it still needs insane amount of hacks to make the layout work reliable across browsers.
EDIT: The common example when grid and flexbox layouts break is when a child of a child element in the layout overflows.
EDIT: The common example when grid and flexbox layouts break is when a child of a child element in the layout overflows.