The gears and the motors are fine. the problem is the case material. It is soft aluminum and has a tendency to move when the gearbox is under load. The movement can be in the motor and pinion moving away from the first driven gear and therefore slipping or skipping. The second problem is the final drive can also do the same thing. If you can brace the gearbox frame or replace it with something stiffer then it would be pretty good. the Asiatam boxes use the same design but they use a steel frame and therefore it is stiffer and less prone to allowing deflection or gear-shaft sideways movement. The design could be improved!! I think the QC question has more to do with the shaft collar grub screws being tightened and secured correctly.