Apple once thought carrying your life (your Mac OS X home directory) was a good idea when they publicly announced "Home on iPod". They quickly reversed course. I assume the failure rates of the iPod's spinning hard disks were a contributing factor.

I think this could only work if you had great backup with the cloud.

"Your iPod wasn’t designed to be used as a startup drive for long periods of time. When the tiny hard drive inside the device spins for hours on end—as it can if you boot from it—the player heats up enough to make your palms sweat. Since there’s no way for the heat to vent, as it would on a hard drive in your computer, you risk tragedy." - Macworld.