I use Illustrator for UX design, and thus I use pixels for the dimensions. However, the resolution of most modern screens, in mobile phones, and other devices, is not the default 72 ppi. Thus, when I switch between units, the pixel size does not equal what I get in millimeters. (For usability issues, e g mm based letter height is crucial.)

I know Photoshop is more of a pixel oriented tool, but doing vector graphics and using Illustrator is an advantage in terribly many places. (There is a reason for Sketch being a popular replacement...) Adding a way to customize the units or customizing the pixel resolution would benefit me and my entire team of 15 people, very much.