We've been through the Consumer Reports argument before. For all the brands and models there are a huge number of samples, which is a basic of research. However, for any one model, brand and year the sample size can be small enough that a few negative or positive samples can skew the results enough to show up in the conclusions. CR does have a "floor" for reporting results but it still may be low enough to affect the results.
It's not all that different that the CPSC issuing a notice. If there were a dozen gas tank eruptions in a model in a year out of thousands of vehicles, there would be a recall. If there are thousands of successful M-Class owners and a few that have had problems are the ones who fill out the survey, the results will be skewed.
We have owned our 2008 ML320 CDI since new and for nearly 70,000 miles with very little problem. Even got 65,000 miles out of the original tires.
I would buy this one again, even though CR does not show it as reliable.
Jim W.