unfitness vs unsuitableness

unfitness and unsuitableness both are nouns.

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

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

unfitness: the quality of not being suitable
unsuitableness: the quality of having the wrong properties for a specific purpose

Other hypernyms of unfitness include ineptness, unsuitability.

unfitness (noun)	unsuitableness (noun)
the quality of not being suitable	the quality of having the wrong properties for a specific purpose
lacking the power to perform
poor physical condition; being out of shape or out of condition (as from a life of ease and luxury)