There were significant engineering flaws in both cars that were huge mistakes by Mercedes Benz. Most notably (and it was an issue with both cars) was the air-ride suspension system Mercedes uses. If there's anything that goes wrong anywhere in the system, the car becomes un-driveable. The whole car settles down on the fenders, and you're unable to turn the steering wheel without shredding a tire or ripping off a front fender. The only option is to call a tow truck and have the car towed in for repairs.
I had my air-ride suspension system fail 6 or 7 times over the life of the two cars. A $12 valve goes bad in the right rear suspension component? Your car immediately becomes a two ton paperweight.
Also, while the build quality is very good, unless you pay a lot extra for all the option packages, most Mercedes Benz cars come with fairly stripped-down features. I had a 2009 E350 at the same time as I owned a 2008 Accord. The Accord had more luxury or optional features than the Mercedes, and was less than half the cost.