I've taken my 190e, VW Passat and corvette(just brake jobs) to the same mech for four years, I don't know if he's certified or factory trained but he's always fixed my car problems and never given me an incorrect diagnosis. The problems have also never come back.
I'm not too fond of dealership service and I've been to lots of different ones from Volkswagen, Mercedes, Chevrolet to Honda, what I've got from them is insensitivity, indifference and high prices for simple jobs.
I've also noticed a subtle kind of reluctance to work on your car if
it's an older model, you get a lot of responses along the lines of, this problem is not worth fixing. This has made an impression on me that if it's an older model take it to an independent, most of the time you won't see a lot of older cars at the dealer anyway but you will see them at the independents. Of course this might just mean that drivers of older cars find the prices of independents more affordable.