niceness vs difficultness

niceness and difficultness both are nouns.

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

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

niceness: 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 niceness include difficulty.

niceness (noun)	difficultness (noun)
the quality of nice	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
the quality of being difficult to detect or analyze
a courteous manner that respects accepted social usage