It is my belief that the failed pumps are a result of improper winterization. Overboard discharge is not common in the US. Most people claim there is no need to winterize it since it was used. However the discharge line from the holding tank goes from the bottom of the tank to the pump. Gravity fills this. Winterizing the tank doesn't displace the contents of the discharge line between the tank and pump.
When winter freezing occurs, the contents freeze, expand, then the seals are compromised.
To properly winterize the tank, the contents need to be either pumped or dumped. Then filled with propylene glycol (antifreeze). Then, when the boat is hauled, I place a large bucket under the discharge and discharge until antifreeze is discharged. This will ensure no complications in the spring.