If had about equal amounts of bad service to okay service (nothing I’d call ‘good’ though), in my costly dealer servicing experience of 2 M5s and a Z4MC.
In some of my cases, when I questioned the 200% difference between a non-M and M service, I was told it was because a Master Tech has to work on the car. But when I got it back and the oil cap was not fitted back on, I was told that anyone could have done it and the Master Tech just signs it off. The service advisor/manager just couldn’t undertstand my concern that I was paying a premium, and that an apprentice & Master Tech had both fooked up.
So I’d stay away from main dealers, as most of their experts on the S54 will have left and set up on their own, and I doubt BMW are providing an further training on the S54 as the youngest engine will be 10 years old.
The only reason I’d take it to a dealer was if I didn’t have a choice, through breakdown or warranty restrictions.