look on youtube, and see all the brushless powered tamiya lunchbox's. they have all plastic geared tranny, yet have little issue with a brushless motor on lipo. but the plastic is good and the gears are thick
if the right plastic is used, and the gears are thick enough, then they will hold up just fine.
BTW, the all steel gears in the rustler vxl........i chewed them up once in mine.
metal SHOULD be better than plastic, but again it matters WHICH metal is used and how thick the gears are.
use the wrong type of metal or plastic, and dont make the gears thick enough(which spreads the load) and they will fail.
i just bought a bunch of models this week, otherwise i would pick myself up the brushless buggy. maybe in a few weeks i can get one.
while i would prefer to see metal gears, im not that worried that the plastic ones wont work well.