subtlety vs difficultness

subtlety and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
subtlety	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficultness is a hypernym of subtlety; that is, difficultness is a word with a broader meaning than subtlety:

subtlety: the quality of being difficult to detect or analyze
difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of subtlety include difficulty.

subtlety (noun)	difficultness (noun)
the quality of being difficult to detect or analyze	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
a subtle difference in meaning or opinion or attitude