Sorry I'm late - I had a heck of a time getting to work this morning, dodging and weaving around all the VWs, Audis, SEATs and Skodas broken down in the road. Oh wait, now I remember - not a single one.
There are millions of VAG cars happily running long drains on Longlife III oils. I have done it myself, and on a BMW (using LL-04). Anecdotal stories on an oil enthusiasts' forum can not be a proxy for global reality.
The oil specifications are intended to offer confidence that the oil can do what the industry body (eg ACEA) or OEM expect it to. The tests are real engine tests, so the oil goes in virgin but gets used during the test... just like in the real world. The tests are made severe to get to the results quicker, since there's not time to wait 2 years to see if an engine wears out or not.
And the same tests are used across various specifications - in back-to-back comparison, a ACEA A3/B4 oil has a lower performance target than an ACEA C3 does. And the C3 oil has to do it with an ash/TBN constraint (this is the only part of the specification that is intended to protect DPFs: ACEA C3 has 14 laboratory tests and 9 separate engine tests covering a range of performance attributes.) VW add their own DPF blocking test, so ultimate ash level is less important that real-life DPF blocking propensity (although there are ash limits within the spec).