i fitted the metal one myself, it was a 3rd party product,
the metal impellor will not physically break, if anythings to go on a waterpump barring the plastic version it'd be the seal or phospor bushing, which if seized would strip your cambelt anyways,
i wouldnt personally worry about it, a lot of water pumps are plastic impellored, a big factor that causes an impellor is the incorrect anti freeze mix in the winter, in which the coolant can freeze and causes damage once the engine is fired up, = bang dead waterpump