Reliability of Samsung phones

Is it a reliability problem or just that the phone Angela bought is a lemon? The phone failed again and we had to send it back to the service centre. That means it is the 4th time we have been to the service centre in a week. They told us that the main board could be faulty therefore they need to get it replaced subject to the availability of parts. Since it was such a big problem why did they not change the phone right from the start?