Worst "feature" I've ever seen added to macOS. Such a waste of potential, because throughout the years we've had different concepts of stacks floating around, but this is truly the worst. Grouping by filetype on the desktop is useless but even if it were useful, the interaction with stacks is abysmal. There is only 1 reason why they implemented this, which is to keep the desktop "clean", but usability be damned.
Grouping needs to make sense by the type of project I'm working on, if I work on a Powerpoint I might have some JPG, some PDF, some MPG files related to that Powerpoint, wouldn't it be nice to make a stack with those related files?
Another cool use for stacks that would have been awesome: lets say you select a bunch of files in a Finder window and drag them to the desktop, as soon as you drop them instead of cluttering your desktop, a stack would be created automatically. That would have been awesome.
But no, mojave stacks is just ridiculous, I couldn't believe Apple had done this when it was announced, it completely made me lost faith in the UI vision for the legendary macOS Finder.