This is a topic debated to death between Mac and Windows users. Here is the argument in favor of the Mac/Ubuntu solution. The time it takes to hit a target is inverse proportional to its size, according to fitts law. A menubar at at the top has infinite height, thus it is a really large area to hit. In practice that makes it much faster to hit a menu bar at the top than inside a window.
What you don't want is an autohiding menu bar. When it autohides you can't see the location of the target before you move your mouse. For people who use this setup are used to throwing the mouse pointer up quickly to the target. You can't do that if the menu is hidden. Then it is a two step process. First throwing the mouse up to the menu and then making a selection.
This is the reason why we keep all frequently accessed GUI elements on the corners of the screen. These are the quickest places to hit. E.g. the Windows start button is fast to hit for this reason. But to be fair you are using the application menu bar a lot more frequent than the start menu.
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.
What you don't want is an autohiding menu bar. When it autohides you can't see the location of the target before you move your mouse. For people who use this setup are used to throwing the mouse pointer up quickly to the target. You can't do that if the menu is hidden. Then it is a two step process. First throwing the mouse up to the menu and then making a selection.
This is the reason why we keep all frequently accessed GUI elements on the corners of the screen. These are the quickest places to hit. E.g. the Windows start button is fast to hit for this reason. But to be fair you are using the application menu bar a lot more frequent than the start menu.