Monoperms, Decaperms, Hectoperms were the BMW/Merc of the motor world in the 1970/80s.
They are motors that if run in properly, still cannot be beaten for reliability - truly fit and forget.
too many have been "raced" and the comutator scarred and windings burned - these are the ones who get criticised.
Personally I buy old ones and refurbish them so I generally can get a good motor for £15-20. Once rebuilt I run them at half speed for six hours in both directions from a battery, or a digital train controller, never an old rectified power supply. The result is a very smooth motor.
Some of the new motors nowadays are as good or better, its all a question of preference, run badly new motors are just as problematic.
Personally for fast (not scale) I use brushless.