When a quad loses one rotor, the opposite one cannot be used for lift without flipping the whole thing over, so you have a bicopter with half the lift available. Engaging the third rotor generates a yaw imbalance making the aircraft rotate. At best you are going to have a controlled crash of a spinning aircraft. A delicate gimbal will likely not survive. It has been demonstrated, in the laboratory that it is possible to land a lightweight quad with a rotor loss. With a heavier quad carrying a camera, you are going to fall in the lake.
With a hexcopter, one rotor loss means you now have a quadcopter with two thirds of the lift available. Having 4 rotors, you can still control yaw.