It’s not that you can’t work in mm it’s just that if you make complex shapes in mm you will be creating geometry that is smaller than the tolerance built into sketchup.
The program was originally designed for the architectural industry, long before 3d printing became such a common option, so very small and also very large things cause issues. General day to day measurements work correctly.
So to get around the tiny face issue you can use what has become known as The Dave Method (there are example of tiny failures in the thread.), or by working at a larger scale and simply exporting it so that is seen correctly in your printing software.
Generally for small stuff it is easiest to use m as if they were mm. No issues with trying to work out multiples of anything, just use 1m as 1mm and away you go.