incapability vs unfitness

incapability and unfitness both are nouns.

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

As nouns, unfitness is a hypernym of incapability; that is, unfitness is a word with a broader meaning than incapability:

incapability: the quality of not being capable -- physically or intellectually or legally
unfitness: lacking the power to perform

Other hypernyms of incapability include inability.

incapability (noun)	unfitness (noun)
the quality of not being capable -- physically or intellectually or legally	the quality of not being suitable
lack of potential for development	lacking the power to perform
	poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)