Most passenger car towbars are rated for a supported load of 50kg (smaller cars) to 75kg (full size like a Merc.).
That tells you how much load you can carry on any type of rear carrier, although you have to take off the towbar supported load already as the (bicycle) carrier often has considerably more overhang.
Do yourself a favor and drive a car with a heavy trailer for a day and you'll realise quickly that overloaded trailers, roof racks AND rear carriers are NOT funny.