Maybe call it intuition, but my sense is that a composite impeller will undergo only so many heat-up / cool-down cycles before fractures develop. The Duratec V6 Ford / Mercury from the Contour / Mystique of a number of years ago suffered waterpump failure / overheating (it being an aluminum heads / aluminum block engine) when the OEM plastic impeller started spinning freely on its shaft, or when the impeller grenaded. I did not let it come to that... rather, I pre-emptively replaced it with a metal impeller jobber water pump...