how tough is a A518

No argument about the Chrysler OD series of transmissions and their problems. I sold a lot of parts for these. Oh yeah, also sold a ton of parts for the Ford series of OD transmissions, and also, the Allison that GM uses has issues also. The common thing we found in the parts/converter industry is what these vehicles were being forced to do. Horsepower & torque is not hard to find in a diesel. The owners were upgrading the horsepower/toque in their MOPAR/FORD/GM diesels and then wondering why the drive train was failing.
Any of you that have actually worked in the transmission industry as a builder, shop owner or in the replacement parts business know what I'm saying. If you want to go and see what has been produced for the diesel owner with an automatic transmission take a look at www.tcsproducts.com and see what they build to help keep these transmissions alive, and they aren't the only one building these HD parts. These companies don't spend a lot of cash in research and equipment because they thought they might sell one or two of the failing parts.
The early design planetary in the overdrive was also a problem area, but once changed to the later design the problems virtually disappeared.

These transmissions fail no more or no less than any other transmission.
I'd like to have a dollar for every 700R4 & 2004R that were swapped out for a TH350 over the years. I'd be a multi-millionaire.