The "problem" with unique is that we don't really associate it with the idea of being the only one. We often think of it in terms of "really special" or "really different." Words with precise, emphatic meanings often weaken over time. For example, in Old English, soon meant "now."
I have to admit, though, I don't really see the problem with "extremely historic." Well, I guess it sounds sort of silly and unnecessary, but I don't have much of a problem with something being more historic than something else.