In my experience, most businesses you're likely to patronize will accept cards - restaurants, shops, supermarkets, whatever. The main exceptions would be probably kioscos/mini-supers (i.e., the small mini market-type stores where you can buy alfajores, candy, drinks, and the like.) If I remember correctly, it might also not be possible to use a card to buy/top up a SUBE transit card. And yeah, if you're buying food from a dude grilling meat on the street, he probably won't take cards either.

If you use a foreign card, you'll get whatever the rate is (and this being Argentina, maybe it'll change!) But cards are certainly accepted in most places - that's how locals pay!