onerousness vs difficultness

onerousness and difficultness both are nouns.

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

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

onerousness: unwelcome burdensome difficulty
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 onerousness include difficulty.

onerousness (noun)	difficultness (noun)
unwelcome burdensome difficulty	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result