I'm not an engineer, but I suspect it has to do with track cars actually making grip, overly aggressive clutch setups, aggressive and much more frequent upshifts/downshifts, and probably a big part of it being sustained high gearbox temperatures.
There is another forum member that has run one of these things as a w2w car for a while, he told me he has been through over a dozen transmissions and said that once he added a transmission cooler it slowed their death significantly. I've been meaning to add a temp sensor to mine, as well as a cooler, just on the backburner.
I do have data from a spectacular 4th gear failure on the FA/Harrop setup, but the Jack's 4th gear failure was pretty silent and caught before a big bang.
There is another K swapped track car (this is not the same person that has been through transmissions mentioned previously) who found one of the three synchro hub pieces stuck to the drain magnet and sent me a picture a couple of weeks ago, so he caught that before the big bang too.
Basically, if the car is kind of difficult to get into 3rd or 4th drop the fluid. If you find one of these:
Pull the trans, it is done, you caught it before the big one.