Well I was a tech for 7 years, 6 years at Mercedes Benz, working at MB taught me a lot esp... AMG cars v12's bi-turbo's, superchargers, complex dynamic seats, ABC hydraulic suspension, & CANBUS systems- (high/low level signals) the technology I worked on was light years ahead of what is currently available at GM, can you image 10 years later!!! GM is just now getting some of the tech that's been available.
To me GM products are like Legos, friends come over to the house and like are you are crazy would you do that to a brand new truck..... , but I prefer the simple machines less failure, a quality product over its useful life time, that's less failure prone. When I see a GM owner, I see a smart costumer, esp... the lower cost of ownership.
With that said, there are few good techs, hence few (I always got stuck with warranty BS work, rattles, suspension noise's) as they don't make money b/c they care to take their time and not make a mess (finger prints on the head liner is a no no) / or want to do a quality job, the first 4 years I flagged 42.5 hours a week management told me increase my hours or they would let me go, I told them to F off (it went against my moral believes of just doing a good job and not screwing over the customer). well I managed to survive 2 more years. I have had enough sense to move on, now I work on stuff at a personal level and for close friends only. On the flip side I worked for a very high line local dealer, everyone here knows their name, sadly I worked with messy crooks everyday. I will admit I am not perfect I did make mistakes along the way but I learned from them.
I have seen lots of auteurs work on their own stuff, mostly to save money, along the way they tell themselves they can do it better work, few can. I always volunteered for the annual MB club to come in the shop and work on their own cars, the guys on the forums (yes, that applies to all of us on here) You could see in 5 minutes all the mistakes/safety issues, even when changing out brake rotors. I have a very advanced level of training for automotive gas/diesel and even became a self taught HP tuners user to learn more about engines vs software, help me diagnose better, to know what the ECM wants to see is very helpful.
Sorry you had a bad experience, I know the feeling all too well. When I take my GMC truck in and I get treated like crap. Good thing is nobody can get anything by me, the dealer respects me but they also hate me as well. Can't BS me on anything automotive related.
Lastly, larger GM dealers have specific line workers, meaning they focus only specific tasks, Interior/trim, powertrain, HVAC, or brakes/suspension. so your vehicle maybe passed around by a lot of techs for a few complaints or repairs and it only takes 1 bad apple to ruin your experience (I was bumper to bumper tech, it came to me, I worked on every issue)
some advice, it doesn't take long for a customer to find out if their service advisor is full of it (if you don't like your current one, ask for another), then once you have a good advisor ask him for the his best tech to work on something, he will only know 1-2 techs.... trust me. The better techs will more likely be at the Cadillac dealers for obvious reasons.
rant over carry on