I'd never heard of a universal motor so had to check Wikipedia....
"... A negative aspect is the maintenance and short life problems caused by the commutator, as well as electromagnetic interference (EMI) issues due to any sparking. Because of the relatively high maintenance commutator brushes, universal motors are best-suited for devices such as food mixers and power tools which are used only intermittently, and often have high starting-torque demands...."
Seems the main reason not to run such a motor continuously would be that they are not so efficient considering brush-wear....
learn something every day.....