1. I made upgrades from 8.1 to 10 and never had any problems. So why that "you must made clean install"? It's probably because Enderman is a person who must write his standard answer as fast as possible everywhere. And his answer is always "you must made clean install".
2. Lot of people use Rufus. There is no difference what program puts iso on pendrive, but I assume Enderman as always trying just one method and this method becomes the only proper one.