I've never owned a plummer or seen one work so can't comment from personal experience, but they're bound to be exactly the same as all other breeds and types of dog, insofar as some will be better than others. If you're tarring the breed as a whole with the same brush based on one individual you've owned, yours is bound to be a very biased view.
ive owned a few and its true some are better than others. however if you source good working stock they are generally exellent bushing dogs .