punishing vs backbreaking

punishing and backbreaking both are adjectives.

Word	Noun	Adjective	Verb	Adverb
punishing	No	Yes	No	No
backbreaking	No	Yes	No	No

As adjectives, punishing and backbreaking are synonyms defined as:

punishing and backbreaking: characterized by effort to the point of exhaustion; especially physical effort

Other synonyms of punishing include arduous, grueling, gruelling, hard, heavy, laborious, operose, toilsome.

punishing (adjective)	backbreaking (adjective)
resulting in punishment	characterized by effort to the point of exhaustion; especially physical effort
characterized by effort to the point of exhaustion; especially physical effort