My dad had a 928 (1979(Manual)) and my grand-dad has a 928 S4 (1988).
My dad put ~330 000km on his from 1981 to 1993. Engine and Transmission without problems. The front suspension was always a problem.
Regular maintenance was changing the bushings in the front. The weissach-axle in the back was fine.
Although it chewed trough tires at an enormous rate. New tires every 20 000km!
The mechanical fuel injection system had to be rebuilt after a few years.
The 928S4 (Auto) that my Grand-Dad own`s hasnt had problems with the suspension or gearbox. But with the ECU.
A few of the sensors went bad, especially the mass air flow sensor. Thats a platinum wire in front of the throttle body.
It's a heated platinum wire thats gets colder the more air flows trough the intake. So in 2013 the thing snapped and trubble was ahead.
Finding a pristine example of such a device was near impossible. Finally he found one in Holland.
Then the entire fuel system clogged up with some mysterious goo.
After lots research I stumbled upon that high performance gas (Shell 100 Octane) attacks the plastic liner in the fuel tank.
This goo then gets into the fuel system through the fuel pump. Its sort of a semi liquid, so only some of it gets absorbed by the fuel filter.
The rest blocked the injectors. So the entire fuel sytem had to be pulled apart and cleaned.... and the fuel cell replaced.
The early 928 are mechanically very complex cars, with some issues. But those issues got fixed during the years. The later one mostly have problems with electro-mechanical stuff.
But wonderfull cars to drive according to them... I want one aswell...
Greetings
Max