The fact that they are using a dedicated app makes it less transparent than if you were purchasing from a web site. At least on the web, you can see the link URL, the domain, and if it is encrypted. All of that is hidden by using the desktop upgrade assistant app.
If it were done on the web it might be one of those deals where you get the message "you are now leaving the Microsoft website..." yadda yadda when you make the purchase. Which I can imagine being more irritating than the way it is with the desktop app and the Arvato disclaimers therein.
So basically, nobody has had any problems with using the upgrade assistant/Arvato to buy Windows 8?