My W211 e320 D which has covered 170 k is maintained by my local indie but when I bought it a year ago I took it to our local MB specialist with 50 years of experience to sort out some problems. Whilst replacing the alternator which is a difficult job, being situated deep between the V6 they noticed a faulty turbo gasket and failed heater plugs . They also have access to the STAR software but essentially it’s their knowledge with dealing with just one marque that counts. German car specialists have to deal with VAG too so they don’t see quite as many MBs .

....was presumaby intended to outperform VW's good old n/a 444bhp W12, which has very few problems despite being a considerably older design.

Certainly the AMG motor essentially being a hand-built, limited-run special design doesn't help. I do tend to think that a well-thought-out design that's mass-produced on good tooling is perhaps likely to be more reliable than a motor comprising lots of special bits (and component replacement costs will be cheaper too).

The example of the corroded / cracked throttle body mounting plate was very surprising. Poor material selection and design..

I'm also surprised the M156 had the very basic design of cam lobes directly contacting the valve lifter bucket (which are known to seize in the M156 - they are supposed to turn a bit each time) rather than using roller-rockers which seem to be a better design - as long as they are made sturdy enough (Audi had many problems with them).