That's no guarantee. My friend had a brand new air-cooled 911 back in the day. He took it in for the first service and although he couldn't see the work being done, he could look out of the waiting room and saw the mechanic start off on a test drive. In a little while he saw the mechanic pushing the car back to the service area and saw him pouring oil in the engine. Obviously he drained the oil but forget to add oil before driving the car. The dealership refused to admit their mistake and swore up and down that nothing had gone wrong. The service was done in Kansas City and my friend was on his way back to California where he lived, so he was in a bit of a bind. They refused to give his car back until he paid for the service, which he reluctantly did. The car burned oil for the remainder of the time he owned it; however, it never did self destruct.There was a follow up thread somewhere here where he asked for the transmission oil brand. According to that everything was resolved when the transmission oil was refilled - allegedly no damage.
To me the lesson from this is to never buy a used car that does not come with dealer service records.
The point being, that getting service at a dealership is not always a perfect experience, But your point is made because there is an even greater chance that the DIY guy will screw up.