Sorry to hear about your experience, OP.
What you are experiencing can apply to almost and dealership/marque .
I had great service for years from a MB dealership but last few year, not the same.
I put it down to the people that work/lead at the place and the chap that
was my service manager was good he left.
I've had problems where the batter on key fob was not changed. I know that for
certain becuse I made sure I sent in the key that would not open and close the windows via key fob as this happens
when the battery is low but it opens the doors and tailgate. Rang them and won't go into it but
they made me feel as though I was lying. I'd paid at the time a good 600 quid for a B service a couple of years ago
and the battery costs a couple of quid. I lost faith and wondering what else they had missed as I was told the "boe has been ticked."
I told them that when I changed the batter over from the key i kept behind to the key sent to them the windows opened. Importantly I've
had a MB close to 20 years and whenever a battery is replaced the second battery as they use a 2 pack batter, the 2nd one is given, sent over with the car and that was not the case. I told the they made me feel like a liar. I took it further and they apologised fr the tome of their email, etc.
and offered to recheck the whole service, I thanked them and declined. I used another MB dealer the next year. However, more recently gone back to them as they are local even though I've had the car collected and returned to me.
I've had our other cars oil changed at Halfords recent years and change of plugs and brakes. The service manager I deal with is a good bloke and because I trust him rightly or wrongly, cars always comeback good.
Things can go wrong it's how they address them. I put it down to the person at the top just like the school I went to. If you have a rubbish person leading, sooner or later the whole place starts to go under.