unmercifulness vs mercilessness

unmercifulness and mercilessness both are nouns.

Word	Noun	Adjective	Verb	Adverb
unmercifulness	Yes	No	No	No
mercilessness	Yes	No	No	No

As nouns, unmercifulness and mercilessness are synonyms defined as:

unmercifulness and mercilessness: inhumaneness evidenced by an unwillingness to be kind or forgiving

unmercifulness (noun)	mercilessness (noun)
inhumaneness evidenced by an unwillingness to be kind or forgiving	inhumaneness evidenced by an unwillingness to be kind or forgiving
	feelings of extreme heartlessness