Although you can set ArcGIS Pro to metric units such as mm or cm. Most of the layout elements like map frames, legends, or text, have to be manually adjusted to sizes and dimensions that would make sense for any logical metric user. No one is going to use 3.473 mm or 6.11 mm. I understand that some of the defaults come from directly converting inches, or fractions of inches to their equivalencies in mm. That results in very unnatural measurements, especially when using A4 or A3 paper.
Unless users has some very precise and specific needs, 99.9% of them would be happy if 1 mm was the set interval when positioning boxes around the page, or when resizing manually. With text is the same story. I set text in mm in options, and I see the defaults being just bizarre, with 3 or 4 decimals of mm, pretty absurd.
Other thing that bugs me is the inability (as far as I know), to set the scale bar to say km instead of the full word kilometers by default. Why?