rough sledding vs difficulty

rough sledding and difficulty both are nouns.

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

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

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

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