onerousness vs heaviness

onerousness and heaviness both are nouns.

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

As nouns, onerousness and heaviness are synonyms defined as:

Other synonyms of onerousness include burdensomeness, oppressiveness.

onerousness (noun)	heaviness (noun)
unwelcome burdensome difficulty	the property of being comparatively great in weight
	used of a line or mark
	unwelcome burdensome difficulty
	an oppressive quality that is laborious and solemn and lacks grace or fluency
	persisting sadness