They even fail on external-drum, downtube-mounted shift levers. But in that case, the impending failure is readily visible to the rider (and individual failed strands alert the rider by poking into their fingers when shifting gears). Drum diameter is inversely proportional to cable failure; the larger the drum, the less likely it is to precipitate failure. Perhaps those MTB and non-Shimano shifters use larger drums than Shimano?