You do have a point, you can't see the menu-bar items when they're hidden, however, I used Mac OS for work in the past, and the full-screen mode (in which the menu-bar does indeed auto-hide) is actually great. Even though you can't see the buttons, I was able to quickly memorize the rough location of what I want to press, so it didn't actually slow me down that much, even in heavily menu-reliant applications.

I feel like this would be even less of an issue with Ubuntu, since it has the menu-bar search feature, where you can "press" a button by typing it's name rather than looking through the menus.