【Onerous】 , 【burdensome】 ,  【oppressive】 ,  【exacting】  are comparable when they mean imposing severe trouble, labor, or hardships.

All of these terms are applicable to a state of life, its duties or obligations, or to conditions imposed upon a person by that life or by another person;  【oppressive】  and  【exacting】 are applicable also to persons or agents responsible for these difficulties.

【Onerous】 stresses laboriousness and heaviness but often also implies irksomeness or distastefulness.

【Burdensome】 usually implies mental as well as physical strain and often emphasizes the former.

【Oppressive】  adds to  【burdensome】 the implication of extreme harshness or severity; it therefore usually connotes the unendurableness of what is imposed or inflicted, whether by nature or circumstances or by man, or cruelty or tyranny in the one responsible for the impositions or inflictions.

【Exacting】 , like  【oppressive】 , implies severity of demands, but otherwise it differs because it commonly suggests rigor, sternness, or extreme fastidiousness rather than tyranny in the one who demands, or the tremendous care or pains required of the one who satisfies these demands.