Perhaps the most apparent is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of 
the tooth mesh. Sound is also suffering from gear and housing components and also lubricants. In general, expect to spend more for quieter, smoother gears.
Don’t make the error of over-specifying the engine. Remember, the input pinion on the planetary must be able deal with the motor’s output torque. Also, if you’re using a multi-stage gearhead, the output stage should be strong enough to absorb the developed torque. Obviously, using a better motor than necessary will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque can be a linear function of current. Therefore besides safeguarding the gearbox, current limiting also defends the motor and drive by clipping peak torque, which may be anywhere from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are simultaneously in mesh. Although you can’t really totally eliminate noise from such an assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form of electric motors. Therefore the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are usually more expensive than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In this kind of applications, the gearhead could be viewed as a mechanical springtime. The torsional low backlash planetary gearbox deflection resulting from the spring action increases backlash, compounding the effects of free shaft movement.
Servo-grade gearheads incorporate many construction features to minimize torsional stress and deflection. Among the more common are large diameter output shafts and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads have a tendency to be the most costly of planetaries.
The kind of bearings supporting the output shaft depends upon the load. High radial or axial loads generally necessitate rolling element bearings. Small planetaries could manage with low-price sleeve bearings or various other economical types with relatively low axial and radial load ability. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like the majority of gears, planetaries make noise. And the faster they run, the louder they obtain.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are generally limited by about 50:1 and up, planetary gearheads expand from 3:1 (solitary stage) to 175:1 or more, depending on the amount of stages.