The OEM and Aftermartket Gates Water pump were not heavy at all. Likely aluminum bodies. The OEM has plastic impeller (did not check the Gates unfortunately) but unless there's hard objects in the cooling system I can't see the blades chipping.
BMWs were notorious for cheap plastic impellers. Actually Euro water pumps in general aren't as good whether plastic or metal. JDM cars fare better.
The water pumps we use in my industry all use plastic composite impellers that are absolutely superior to the old metal ones. Plastic is not inherently bad and any product can be poorly made, metal or plastic.