Can't comment too well on how well the pumps are holding up, but the bearings wearing out is probably more of an issue with the bearing design itself as opposed to the materials of cosntruction of the pump. Aluminium is rarely used as a pump material for such applications. What perhaps could be considered is stainless steel or wear resistant cast iron like NIHard. Normally the castings are manufactured as an integral piece and not in segments as mentioned by you. This could be due to pump characteristics.
Plastics are neve talked about for such applications.
As regards corrosion near the bearing surface this could be handled well by proper maintenance. Leakages and sealings will have to be considered. If some portion has worn out perhaps rebuilding will have to be considered. Makes for an inexpensive product that will last through the warranty period then fail. Shades of planned obsolescence and continuous spares replacement.