I don't eat potatoes, I've only made them for others at Thanksgiving so I just used a fork, but I think the people who prefer a masher say the mixer overbeats them.
It is liable to turn the potatoes into a purée and into a liquid which is absolutely terrible to serve with anything. It basically turns it into potato soup.
I remember I only made that mistake once years ago. I got lazy one day and decided to use a hand blender to mash some well cooked potatoes.... it only took ten seconds to make potato soup. Then I had to make another batch of potatoes after.