I'm sorry to hear about this discouraging message about Ah B. I don't blame you coz maybe its your first time visit. I've known Ah B all my life and he's been in Merc line since the very beginning. That's about 20 years ago. He may not have much knowledge on the newer models but as far as W210 and below until W115, he pretty much can cover his eyes and fix it.
I think the main reason he's listed in MyBenz is because of his knowledge on the older models and his decent pricing. And he doesn't mind if you bring in your own parts. He might not be as polite as u wish but inside he's a very nice and humble guy.
I've seen A LOT of foremen out there and basically i still see them every single day. AND i can tell you, knowledge on fixing car is not something you can get by just reading books or attending short seminars or even with a cert from C&C. A good reputable foremen is someone with great knowledge based on his years of experiences.
I think with the numbers of client that Ah B gets a day, i think a lot of people would agree with me!