First I do not know any statistics, especially about the x5.
But I think mechanical water pump failures are rare in cars in general.

Now what happens often and would be smart to replace is the thermostat.
When a thermostat fails and it sticks closed the engine overheats and.when bmw overheat.they can blow head gaskets or.crack heads.

Fyi my x5 service records show the prior owner had a thermostat failure at about 75000 miles and it overheated and blew a hose.

If your x5 is about the same miles it would be good to replace it. Also reminds me the x5 doesn't have a temp gauge. I need to get my bmwhat iphone reader hooked up so I can have that info.