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patrick_y
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It's important to note that Mercedes has a lot of emphasis towards long term reliability...

It's important to note that Mercedes has a very large emphasis internally towards long term reliability of the drivetrain.
And they've been doing the 60 degree V6 for many years now, and they already have a lot of experience.
The common problems with the BMW V8s were usually related to seals. The BMW engines were designed to run hot for efficiency, but this took a greater toll on their seals. Combined with a longer oil change interval (in the USA the oil change interval on V8 engines were 15,000 miles) the seals weren't always in optimum oil.
BMW V8s were inherently leaking oil, sometimes from multiple places, but the engines were otherwise perfect. Smooth, eager to rev, great throttle response (until they went to turbocharged V8s), and very powerful.
It's important to note that in general, many V8s on the market today are considered "cutting edge" technology. You're dealing with a powerplant that produces likely around 500 hp. So it's an engine that is cutting edge.
Older AMG engines, M113 non-supercharged (CLK55, C55AMG) and M113K supercharged (E55, SL55, S55, CL55, G55)
One of the most reliable Mercedes-Benz engines is the M113 platform V8. Commonly used until 2006. The AMG variant was the M113K, 5.4L, used in cars such as the E55 AMG that was supercharged to 476 horsepower and 700 NM torque, S55, G55, CL55, and SL55. That was a very reliable engine. Very few problems. Very reliable platform. The 5 speed automatic transmission utilized here was also very dependable.
Naturally Aspirated M156 engine (M159 engine in SLS AMG with wet sump lubrication)
The M156 engine, which was the 6.2 liter AMG bespoke engine suffered primarily from head bolt issues that would allow coolant/water into the cylinders. Since water doesn't compress, the engine would bed rods and fail. Mercedes switched to a 7 speed transmission here and experienced a few problems in the transmission department. These engines are great sounding, fuel consuming, very free-revving especially for an engine with that size of bore and stroke, and incredibly emotional. Long-term owners who are in-love with this engine will keep their cars running reliably by doing a one-time change of the headbolts to an updated design that is reliable. This is a labor intensive process, as half the engine needs to be taken apart to change out the headbolts. In the USA, the cost is around $4,000 USD to have this done as a preventive measure at a dealership.
For certain models, Mercedes-Benz also used a torque converter automatic transmission OR in rarer cases, instead of a propeller/impeller torque converter transmission (think two fan blades, one connected to the engine propelling oil into another fan-blade impeller that is then turning the transmission) it had a wet take-up clutch, basically a multi-plate clutch that would "start clutching" once the engine span above a certain speed (indicating the car was to move forward as soon as engine speeds were above normal idle speed). The term "Multi-Clutch-Transmission" or MCT was used to denote this kind of transmission. IT IS NOT TO BE confused with a dual clutch transmission. It's still a normal automatic transmission, just with a different type of medium between the engine and the transmission.
Turbocharged Generation AMG engines (M157 and M178)
The more recent wave of 5.5 liter V8 turbocharged engines and now 4.0 liter V8 turbocharged engines are harder to ascertain their reliability. The 4.0 liter is predicted to be very reliable and Mercedes-Benz is implementing that engine in cars across their model range. In the non-AMG C, E, S, G, SL, CLS, and probably more models in the future. This 4.0 liter was primarily designed for its reliability and to meet China's standards. China has a very high tax implication on cars with over 4.0 liter displacements. So every brand is strategically designing engines that meet China's requirements. The problem with this latest engine is that the turbochargers are nestled in the middle of the "V" of the engine which makes it very hot there. The engine electronics/computers are also placed very close to the turbochargers on some variants, probably not the smartest idea, and the difficulty of access makes the part harder to service if it's ever necessary. Nevertheless, despite these drawbacks, it is anticipated this engine will be very reliable and the same unit is capable of delivering horsepower from 450hp (C63) to 500hp (C63S) to over 600hp (E63S).
I don't anticipate any major problems with the new 4.0 liter engine, despite the weaknesses I mentioned earlier. It's the same engine being used in non AMG and AMG vehicles by Mercedes, primarily with a higher horsepower output and tuning on the AMG versions. So there is volume, and you need volume to achieve reliability.
A car that has been "forgotten" but was a truly special car was the CLK 63 Black Series in 2008. These had the M156 engine. If I could find one of those, change the headbolts to the updated version, I'd drive that to my heart's content! It's not the fastest anymore, since the new cars are now in the 3-second range. But that is an emotional, race car. It's missing the rear seats and seatbelts though, so your daughter and dog won't be able to come if your wife is with you! But that's a true collector's car.

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