At no time during the upgrade from 19 to 20 was the computer otherwise unusable, and there was only one reboot needed, when the process was complete. It took several tries because of glitches, such as needing to clear enough space for it to work, but each time that happened, the cause of the issue was clear because you could read the error message in the terminal. Frankly, this unofficial method looks better to me than the official one, but YMMV.