The extent and effort needed in keeping a updated system running is directly related to the size of the installed system. If all you got is ~30 carefully picked packages to create a fully custom ISS installation, its not that hard to go through every single line of patches that is needed. This is a huge difference between windows vista/7/8 where such review is not even possible to do and the number of "packages" is astronomical larger.
> Also I wonder what this means for power usage, which is not unimportant in space. Windows still gets longer use out of most laptop batteries than Linux.
I would almost call FUD there, but I will assume good faith. Back at the time with vista, linux system tended to have several times more battery time than windows. After windows 7 was release, and that linux had a power management bug, Ubuntu got a reputation for worse power management than windows.
Now after Linux 3 kernel, linux machine tend to again have much better power management than windows 8. There are some direct variation depending on what distro you install and what additional fluff that is running in the background. Also, EFI BIOS is know for a slightly worse performance for linux vs windows.