For more than 50yrs, RC servos have been vulnerable to localized gear-tooth wear and breakage. A key Bittle design advantage is the spring mechanism in the limbs that cushions gears from gear-tooth shock loads.
Designers typically choose metal gears as needed, but with added weight and cost compared to plastic gears. For multi-DOF robots with the same servo along a limb, an easy cost-structure opimization is to specify metal gears for servos near the body, subject to greater lever forces, and specify plastic gears for servos farther outboard, where forces are less. Mass is also best concentrated near center-of-mass for vehicles and mobile robots.
For some designs, periodicly rotating servos may extend service life, by better distributing wear on gear teeth. Ultimately, RC-format servos are not most-optimal for long heavy-duty service life of locomotion and other extreme load-bearing use.