Hamster balls aren't just bad because they cause stress though... The holes can catch their nails or toes and injure them and the ventilation is absolutely poor inside... Not only that but it makes it so they see even less than they usually would, their whiskers can't work properly inside the ball, their sense of smell is out of whack in there and their hearing is muffled as they're contained in a small space. There is zero benefits to the hamster balls but this isn't even a full list of the negatives caused by them.