Also because it often violates the standard UI conventions for the given OS, so I have to learn how to use every application from scratch.† Accessibility is an

Also because it often violates the standard UI conventions for the given OS, so I have to learn how to use every application from scratch.† Accessibility is another big one. † The flip side is if you use the same app on different operating systems often. There are some apps that fall into that category for me personally, but I think your average user probably only uses two operating systems regularly: one on their mobile device(s) and another on their "real" computer(s).