I believe that the problem lies in the "new" designs on these drives. Used to be the gears were big and wide, but not any more. These gears are now very narrow, and there in lies the problem. the gears can't take the torque needed to handle the weight of the locomotive and a train, so now more and more teeth are being sheared off. I first saw this problem with the first Athearn Genesis steamers, the USRA light 2-8-2, and 4-6-2.
This is why I won't buy a BLI, MTH, Athearn, or P2k, any of the newer steamers, except for Spectrum. At least Spectrum gears are still substantial. If they go to thinner gears, I won't buy them either, unless I can replace them with NWSL gears.
Look at brass steamer gears. They generally have very substantial gears, and very rarely do they have problems like this. Their gears are also easily replaced when and if needed. I own brass locos from the 1960's that still have their original gears, and are real heavyweight pullers.