I'd say they are extremely tough. I've yet to destroy my metal gears and yolks runnin a MM 6900 on 3s, with a fairly tight slipper. Now I have seen one guy have problems with the internal diff gears/top shaft. He was also running twice as much power as any of us, runnin his rustler into the 90mph range. So there is a breaking point to these gears. Extremely high rpms along with a 4 or 5s lipo on a torquey motor along with a little chatter while braking will cause the gears to fail catastrophically. The only way around that is custom machined,heat treated steel gears. Other than that, it's a very well designed transmission that will handle way more than what the vxl on 3s can throw at it. I think it was the planetaries and/or the top shaft we found to be sintered powdered metal, and not a true steel alloy, so they aren't going to be able to take as much punishment.