Well for me it was the plastic stuff that was breaking; gears and track links. You know the story, it's repeated here for the new guys. We (my children and I) don't run our tank exclusively in scale terrain. The tanks are driven through mulch, pebbles, sugar sand, mud, muck, puddles, off curbs, shells, clay, rocks, high grass, weeds, etc.

The final drive plastic gears split when attempting to pivot in sugar sand. The plastic track links broken in all sorts of terrain, even on pavement from picking up rocks and the rocks getting between the sprocket tooth and track. Sprocket always wins! Metal parts eliminate this reliability issue for us.