How do you fix what works when you check it? If someone says something isn't working but when you go to check it, the item is operating normally, how would you fix it? Add to this, you need to convince the person paying the bill that your diagnosis is correct as they don't want to pay twice.
Unfortunately, a lot of issues with cars are intermittent in nature and GM, nor any other company backing a warranty, wants to pay for something on just a hunch. That's why we need to verify the issue, then determine the cause. As a customer, if you know how to duplicate the issue, it's in your best interest to show the service advisor or someone at the dealership the issue and how to make it happen. In cases where it happens randomly, you have to be at least a bit understanding of the situation. It's not good for anyone involved to have to come back twice for the same issue but it is what it is sometimes. It's also very possible you had the misfortune to get a lazy tech working on your car, in that case, and since you've had other experiences already, maybe it's time to try a different dealer if you can.
As for checking the vehicle before delivery, yes there's a procedure but it's not more than checking something to see if it works, if your radio worked when turned on and played a CD, the inspection would turn nothing up. It's not a stress test situation, just a function check.
I don't fully agree with this assessment, especially with the check engine light. Codes are set every time the check engine light goes on, so even if the problem was intermittent, they should have scanned the codes and determined what the problem was and fixed it the first time.
I actually had the same problem with an Infiniti I owned years back. The check engine light went on, I brought it in for service, picked it up and the light was off. But it would go back on again within a week. And the problem was not intermittent. When the light went on, it stayed on. After bringing the car back 3 or 4 times, they finally changed the sensor that was causing the problem. All the other times, I think they just cleaned the sensor and assumed that would fix the problem. There's no excuse for having to bring a car back more than once. They should have changed the sensor immediately - not after the 3rd or 4th time.
As far as the radio turning off, in these times of high technology, I'm sure they can determine what the problem is in a similar way to the check engine light. If there's an electrical or programming glitch, the diagnostic computer they use should be able to determine the problem and get it right the first time.
I think they rush to fix these cars, because they don't want to give you a loaner. My dealership had a policy that if the car didn't have to stay overnight, they wouldn't give you a loaner.
I was lucky that I never had to bring my car in for service with my Malibu, because the dealership was pretty far away. I once had a minor problem that I wanted checked out under warranty (I felt the alignment was slightly off) and when they told me I couldn't get a loaner, I brought it to my local garage and they fixed the problem within 15 minutes.