Is your mechanic a Mercedes specialist?
If so, then I'd trust your mechanic. If it makes it easier or quicker for him to do the engine work with the engine outside of the car, then I'm sure he knows what's best.
If he's just a regular mechanic who works on anything, then I'd wonder.
From reading the techs on this site, I've learned that these cars are really different, and must take years of experience to learn all of the details and knowledge to properly diagnose and work on these cars.
I've also noticed that techs here who own cars of different makes don't do the work on their own cars themselves.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".