Dual monitors is one of those things where the Linux world had an early lead and squandered it by arsing about in a dozen different directions. The pain level seems to have dropped off considerably since we stopped having to use proprietary nvidia drivers, which is good as it means you don't have to google for things with a "-ubuntu" flag as much. I think the last problem I encountered was trying to rotate one monitor 90 degrees turnwise, when the stupid GUI tool only wanted to do it widdershins, easily fixed with the correct command-line invocation.
As ever with *nix, it's a case of once you do finally persuade it to work, it tends to stay that way.
My early experiment with dual monitors on Windows 98 was a right laugh, though: I had a spare old graphics card, and had borrowed a monitor for a weekend, and found that - while primitive - the extra space was a wonderful thing. So wonderful, in fact, that after removing the second graphics card, many windows forever
[1] insisted on opening in the non-existent space somewhere off to the right of the desktop.
I think that got sorted in the Windows 2000 era. These days it's just random icon-shuffling, and occasional weird hot-plugging glitches that get solved by a reboot.
Never seen dual monitors do anything other than Just Work on a OSX Mac, though I'm sure there are projectors that can manage to fuck it up. Projectors are like printers from a higher, less important circle of hell, and connecting anything to an Apple laptop runs the risk of the electrons getting stuck in the compulsory adaptor dongle.
[1] Well, until the next fdisk and OS reinstall, which is the Windows version of forever.