rough sledding vs difficultness

rough sledding and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
rough sledding	Yes	No	No	No
difficultness	Yes	No	No	No

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

rough sledding: a difficulty that can be overcome with effort
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 rough sledding include difficulty.

rough sledding (noun)	difficultness (noun)
a difficulty that can be overcome with effort	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result