used to own many German and Japanese for me and my family, most German we owned were well know for terrible maintenance. But for me, it is not so bad. Parts are cheaper for German car (in Hong Kong, and the labor to fix it was cheap and most the local garage have the equipment computer and tools for specific make)
Common problem I experienced
M.Benz W140 S500, S600, engine wiring, soft close air pump, a/c, ECU, ABS sensor, that's it, other is just wear item like suspension bushing, engine mount. S600 rebuild the transmission twice(every 2 years) for the friction disc. I don't blame the car since my young brother like to burn out with this car whenever he got a chance.
W215 CL500, the common air suspension leak.
BMW E39 540i, E38 740i E39 is almost problem free, the electric shocks lasted 8 years(80,000km) before I swap to Sachs sport kit. E38 have all the problem, light module, ABS, A/C...
Now, I own a 2002 VW Passat with almost 300k km mileage. It still drive like new with proper maintenance.
Japanese I owned do suffer some problem. My 2000 Aristo Vertex have transmission problem. Idle problems, power steering problem. And the garage said they can't rebuild they can only replace with used(new are expensive) and most parts are more expensive compare to the German I own.
ex idle valve on a 2000 BMW 540i (HK$700/CAD $80) VS Throttle body Assembly for a 2000 Toyota WINDOM (Lexus ES300)(HK$5400/CAD $750)
Now I moved from Hong Kong to Canada. And I find out that Euro spec car is different than the N/A spec car. Like BMW E39, Euro spec car don't have emission air pump, EGR... and it is more reliable than the car here.