I know that if someone is to buy a dog and start its training themselves they are more likely to make training mistakes than a professional trainer will. Loyalty has nothing to do with this. An adult dog will be just as loyal to a good handler as a pup is. It's cheaper to buy a pup and raise it. Even though most problems are covered by a good guarantee it still means starting from scratch.
Selection tested dogs are mature males.
If the dog is worth his salt he is going to be a challenge to train and handle. Without proper guidance a new handler will have problems with pack order and control. These issues are easier to settle with puppies as they grow. So this issue is not as clear cut as it may seem. I am more in favor of a puppy program.