I can only speak to our experience. We only ever had one come back - it was for something minor that I'm 99% sure the owner tried to tune himself and screwed up. It was fixed in a day and ready to go out again.
Naturally, we expect people that have issues to come back to us and we'll help them, but sometimes people just want to complain and let their buddy/gun butcher go at it.
Another issue that is out there... ammo. Unfortunately we have to rely on the reloading prowess of the purchaser to ensure reliability since 9mm major isn't commercially available. As a friend of mine once said, "they don't sell ferraris and have to rely on the owner to brew their own gas." Well, with race guns - they do.
Finally if a DVC Open came from us and it has a problem - we'll stand behind it and get it fixed asap and at no cost to the purchaser.