When designing geared mechanisms one very important factor is Torque.
There is a point with normal gear when under high torque conditions - teeth will slip and grind - first reducing efficiency, and also damaging the system.
Another thing is that you will occasionally get play or slack between the gears - this may reduce efficiency, as well as accuracy when using things like Servos or Stepper motors.
One way to overcome these problems is to make the teeth of the gear Helical. This can be done as simply as slightly slanting teeth - right the way up to a full worm gear- which have very little play, allow only a one way transmission, and can deal with very high torques.
Effectively you are increasing the contact between the gears by using helical gears. This may lead to increased friction though - so you need to ensure these are well greased (but you were doing that anyway weren’t you?).
Most professional transmission systems use these.