About How to Use 凤毛麟角
I guess you can see it as something you describe the occurrence of something not directly something. And it's better to be used as a sentence adverb.
People who truly understand quantum physics are really rare.
And the meaning of 凤毛麟角 is "as rare as phoenix feathers and unicorn horns." or simply "very rare." That's why your teacher told you it means unique. However, you should try not to use it to describe particular instance of something. Instead, use it to describe the occurrence or existence of certain type of things.
But when you used it to describe people you can see why it would be sort of weird:
You are as rare as phoenix feathers and unicorn horns.
你真是凤毛麟角。 You are really rare.
Although it make sense in English, it's quite uncommon in Chinese because you are describing a particular individual.
Instead, this is better:
你这样的人真是凤毛麟角。 People like you are really rare.
In this sentence you are describe the occurrence of people like "you".
The reason of focusing on the occurrence of something is because the implication of 凤毛麟角 is something extremely hard or even impossible to find or occur. But when you use it to describe a known instance, you've actually already found it, which is short of against the setup of the scene.
Proper Way to Compliment Other People
If you want to compliment a particular object or person, use following idiom or adjective:
Both of them can means special in a good way. The first one is an idiom which literally means different from others, but doesn't necessarily be the only one of the same kind.
This means unique or unparalleled. Literally it means the only one and there is no second object or people which is of the same kind.
Another term to say people are special or different from others is 另类. But it can be in both a good way or a bad way. And some people what consider it offensive without a context.