The above motor isn't stock, nor is it treated particularly gently. Insofar as I've seen the primary causes of jackshafted ISH failures are old hubs being "recycled" into a build, maladjustment of the shifter or the more common miss shift and/or "speed shifting" (not backing off the throttle before shifting).
I've hauled cargo, raced others & even accidentally speed shifted... overall I'd have to report that the SA X-RD3 is the most reliable component on that build as it's the second motor it's had, it's on it's fourth freewheel bearing (pedal crank) and I couldn't even tell you how many times I've had to rebuild/repair the shift kit (2nd gen) lol
I build my own jackshaft assemblies now https://www.youtube.com/watch?v=U10xLqTekOw
- if it wasn't for the fact that's got an electric hub (hybrid) I'd have happily tried the Sturmey Archer XRK8 8spd disc internal hub, though reports have said the 3sp ones are the strongest *shrug*