heaviness vs onerousness

heaviness and onerousness both are nouns.

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

As nouns, heaviness and onerousness are synonyms defined as:

Other synonyms of heaviness include burdensomeness, oppressiveness.

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