I'm pretty sure that in fact they are. You can't have HoH active in a temple and the art pile. What other reason then that they're unique?
This is correct.
What exactly allow unique artifacts to have facedown copies in play when other card types can't? I still think it'd be easier to remove the restriction of duplicates on face-down cards, for consistency sake.
The difference I can see is that there is no way to force you to activate two of the same artifact, or to flip up a face down artifact while keeping the same artifact active. However, if you have an ambushed Hero who is the same unique Hero as another in your territory, then attacking with the ambushed Hero would result in a situation wherein you would control two copies of a unique Hero.
I honestly wouldn't mind it either way, but that's all I can think of as a reason for the way it is.
