onerousness vs difficulty

onerousness and difficulty both are nouns.

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

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

onerousness: unwelcome burdensome difficulty
difficulty: 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 difficultness.

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