One last technical point:
Clutch choice has a huge influence on the reliability of the gears. This is due to the shockloading that occurrs when the clutch is engaged suddenly. A normal organic disc with sprung center hub and marcel springs will engage smoothly and gradually and the sprung hub will also help absorb some of the shock that would otherwise be transmitted directly to the gears. When you get into racing clutch designs most have been designed for fast and instantaneous lockup for performance and to keep heat generation to a minimum. This increases the shockloads transmitted to the gearbox pretty dramatically.
On the other end of things....tire grip also plays a part because tires that can slip and spin have the same effect in reducing the shock levels as a less grippy clutch would. With AWD systems you have essentially twice as much grip to begin with....when you add sticky tires and a harsh clutch you are just asking for trouble.