Part of the reputation for things going wrong in luxury cars is due to there simply being a lot more *to* go wrong,
That's why I bought the 300SD, and it's also why I bought the A8. The 300SD is as dead-nuts simple as it is possible for an S-Class to be. It has none of the fancy stuff they put on the gasoline models, except a sunroof. It does use advanced materials — it's got Aluminum hood and trunk lids, and it made early use of composite bumper pieces. This was a design completed in 1978! But it has extremely advanced driving characteristics for its age, due to the combination of its multilink front suspension and semi-trailing arm rear that provides controlled toe in response to body roll. When the springs and bushings are in good condition, the whole car completely cooperates with the driver even when pushing it past its limits. Without going into a lot of detail, the A8 is as close as you can get in a more modern luxobarge that has a slushbox, and there's a six-speed manual transmission available for it unlike, for the Mercedes. The AWD is based on a mechanical center diff, for example.
Another thing worth considering is cost/availability of parts...
That's why the W126 300SD has to go, and the D2 A8 is incoming. You can actually get parts for the A8. In fact, my 1998 is a "parts car" that I got for $300 which, except for the failed transmission, is actually nicer than my 1997 for which I paid... more. I hope to recoup some of the costs there by selling some of the more valuable parts from the 1997. The nineties were truly a time at which Audi really only sold one car but in different sizes, and there is a lot of sharing between the A4, A6, A8, and even the VW Passat. (Wikipedia claims that the B3 Passat was a completely distinct design which does not share parts with Audi models. Nope.)
Jaguar parts are easy to source in the UK and relatively cheap, but in other countries they can be difficult to find and expensive for instance.
For large parts, that's a problem. For small ones, I can reasonably source parts from the UK or Germany. I've done both, though I'm not sure I've done both in the course of working on the Audi. I certainly have for the Mercedes.