Pedantry aside, I'd like to suggest that common sense and should always trump grammatical "rules". If we might assume the layperson's definition, "one of a kind" (ie, one instance in a set), and acknowledge that one such kind may represent a radically larger set than another, how might we use English to describe the difference?

The letter "R" is unique in the English alphabet. The particular grain of sand in the palm of my hand is unique to the world's beaches.

Are "R" and this grain of sand equally unique? Might there be a way to compare them?

I don't want to debate the point, rather to pose the question and see what any responses might illuminate.

Have a nice day. :)