This is a matter of semantics, not grammar. It's a cousin to the famed linguistic problem with the sentence "colorless green ideas sleep furiously". Grammatically, it's correct. But it's completely meaningless, and thus bad english.
All words have inherent context and connotations. They have meaning beyond the mere definition and part of speech of the word. For example, idea is an noun. And yes, adjective + noun is syntactically correct, as is noun + verb. But ideas can't have color. They're intangible. And they can't sleep - they aren't living beings. "Green ideas sleep." is perfect grammar but semantically hollow.
"Unique", like everything else, carries extra meaning. In the strictest sense, it's an absolute term, like "pregnant", or "dead", or "same". You're either pregnant or you aren't. Two things are either the same, or they're not. Using quantifiers with absolute terms is meaningless. "These pants are a little bit the same as those." Sounds wrong, is wrong.
The function of language is to carry meaning - if what you're saying has no meaning, it's bad language. Bad English doesn't just mean "bad grammar". So don't try the "grammatically okay" argument, it doesn't hold water.
It has come to be, however, that "unique" can mean something along the lines of "new and different from most other things":
"Ooh, Julie, that dress is very.. unique!"
"CALL NOW ABOUT THIS UNIQUE OPPORTUNITY!!!!!"
"Our unique audio compression technology delivers realtime e-media blah blah buzzword blah"
We already have plenty of words and phrases that mean something like "Not like most others":
..and so on. So, while I appreciate your refreshingly different
use of our language, couldn't you use one of those, instead of trying to force the word unique
into situations it really doesn't belong in? I'm tired of hearing it.