I've always done upgrades and it's been extraordinarily rare that I've had problems. Rare enough to be completely overshadowed by all the extra work it takes to download, install, and configure all the software on my system.
When going from 7 to 8 I had a few issues but I later found out they were caused by roaming profiles -- and they followed me when I reloaded my laptop after getting a new SSD and being unable to clone the old one.
And clean installations can also be much more difficult when you have to go out and find drivers -- especially if you only have one computer and the first driver you need is the one for your network card.
Regardless, users have the choice when going from 7 to 8. Convincing them with verbiage on the web site that they have to buy 8.1 to upgrade from 7 is very misleading and detrimental.