inability vs unfitness

inability and unfitness both are nouns.

Word	Noun	Adjective	Verb	Adverb
inability	Yes	No	No	No
unfitness	Yes	No	No	No

As nouns, inability and unfitness are synonyms defined as:

inability (noun)	unfitness (noun)
lacking the power to perform	the quality of not being suitable
lack of ability (especially mental ability) to do something	lacking the power to perform
	poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)