A slow cooker will not roast. It will only stew foods. A whole small chicken might be possible, but you must add liquids such as a tinful of chopped tomatoes.
I have often stewed cheaper cuts of beef, mutton, chicken legs and cheaper pork chops. Also, coq au vin is good if slow-cooked. Good to add the remains of a bottle of red wine to beef or chicken, but not so successful with pork or mutton. If you want to add vegetables, best add them halfway through cooking. Or, in the case of mushrooms, half an hour before you want to eat.