Not always. I'm not sure whats causing the problems with the gears keep breaking, maybe too much flex in the gearbox or something. If you make fully enclosed gearboxes with the right ratio's you shouldn't have any problems at all. Dave's gearing still is too fast even with the speed 900's in. He would probably get away without burning up motors if the magnets weren't in.

Ploughbot for example uses astroflight motor's. Much smaller and lighter with plenty of power, but being smaller they're easier to burn out. I use them on 15:1, and will probably make the next set at 18:1 as they're still plenty fast enough. I have fully enclosed 2 stage gearbox with TINY gears in. I'm using MOD 0.5 on the first gear stage and MOD 0.7 on the second. I've never had one strip or break or anything and not burnt a motor out on this set up.