The main advantages of a permanent magnet excited DC motor are the high power efficiency and the comparatively low costs of the DC/DC converter, needed for speed control. The speed can be controlled very simply by the armature voltage. The torque is proportional to the armature current, which can be measured very simply. It is possible to manufacture the DC motor automatically, what leads to a cost reduction.
The most important disadvantage is the requirement of maintenance caused by the mechanical wear out of brushes and commutator. The low armature voltage, limited by the number of commutator segments, leads to thick wires in the DC link.
Another disadvantage is, that each motor requires its own DC/DC converter for speed control. This is necessary because each spindle must be shut down independent in case of an error - e.g. the breaking of a filament - and run up after the error is cleared.