Here's something interesting to consider regarding the level of technology used to design the typical main rotor gearbox. Take the example of the AS332 MRGB failure in April 2009. The gearbox was designed prior to 1993. And the accident investigation was performed in 2011, around two decades later. If you take a look at the very sophisticated finite element fracture analysis study work described in the 2011 accident report, you'd appreciate that this level of analysis capability was far more advanced than what was available to the gearbox designers in 1993.
The various types of failure modes and the fundamental process behind each one, have been understood by gearbox designers for many years. But remember that the gearbox conforms to a design that was certified many years ago, and represented the acceptable level of safety and reliability required by government regulations.