could be a manufacturing defect
could be a bearing failure, that stuff happens in high volume production environment
could be a f'up in heat treat, that sintered metal stuff, you gotta cook it just right - or even a chipped tooth from handling pre-heattreat.
Could be some lil kid making $.50/day forgot to put the grease in that one.
could be a bunch of stuff that'd cause the r&p to fail
it's not often, but...jus say'n, not necessarily a design defect
Here's a Losi 5ive compared to a 1/8th scale, I tried to shoot them at the same angle.

Looks to me like the X's diffs are about the size of the 5ive and those are really beefy diffs.
Tell ya what, send me an X and I'll put a 12s 20HP powerplant in it and then we'll see if the diffs are worth a hoot.