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ime - The main objection to a metal impeller is that when the bearing fails it will excavate the w/p housing in the crankcase half. A diligent operator would routinely check the bearing and replace the w/p proactively after about 50-70k miles*Search for more details). It is easy and quick to check the w/p bearing.
The plastic impeller mode of failure is disintegration .The fragments can lodge and block flow anywhere in the cooling system.Good luck finding them. How would you know some vanes have broken? Maybe a cooling problem.Maybe not. Routine replacement may or may not be soon enough to avoid the problem.
There is way more in Search -many more pros/cons
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