there is a 'known concern' with the bell crank lever in the linkage to the inch pedal that if not properly checked and lubed, will cause the inch spool to not return all the way, causing the trans clutches to slip. it is an easy fix/maintenance to completely disassemble the lever and lube/grease the pivot shaft every so often, and then you have to teach the operators that the only reason to use the inch pedal brake has to do with when you need to run the engine faster to lift, and to use the regular brake pedal for everything else and to not ride with their foot on that pedal.
Part of this 'concern' is related to how 'hard' a clunk this trans makes when it engages the trans. This one is smoother and has slightly more travel of the inch pedal as the clutches engage, than in previous trucks, but as a result, the operator may not notice the trans slipping as they bounce down the aisle, but it may well be slipping (clutch pressure lower than normal for a few thousand seconds a week can add up quick).

I have had no problem. works good. how many hours have they run and what about the site condition ?

need to know what is failing, what the environment is, what the oil looks and smells like at failure, if the inching is adjusted properly etc.

Sounds like operator abuse or incorrect maintenance/adjustment if you have multiple units with failing transmissions.