ruggedness vs difficultness

ruggedness and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
ruggedness	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficultness is a hypernym of ruggedness; that is, difficultness is a word with a broader meaning than ruggedness:

ruggedness: the quality of being difficult to do
difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of ruggedness include difficulty.

ruggedness (noun)	difficultness (noun)
the property of being big and strong	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
the quality of being topologically uneven
the quality of being difficult to do