Dogs learn in much more specific contexts than humans do. This means if you teach your dog to sit in the livingroom, he doesn’t know how to sit in the back yard. That’s because he learned what it meant to sit in the livingroom – no where else. If he learns to sit from a standing position, he does not know what it means to sit when you ask him when he is lying down. That is because you taught him to put his bottom down, not up.
So when a dog has been trained else with someone else, your dog has not replaced associations that he has learned with you. Here are some other considerations:
- If your dog is is aggressive towards you or another member of the family, this is often categorized as dominance aggression. The problem lies between the dog and the people he has the issues with. The majority of dogs seen by vets for aggression problems are thought to be dominantly aggressive. If you send your dog away to a trainer outside of your home for training, you will still have the problem when your dog returns 
- You cannot guarantee any what is actually happening to your dog in terms of treatment. Are shock collars, deprivation techniques, treats, or isolation are being used? What about neglect? This is the biggest risk you will face. Best case scenario: are all the people the qualified to handle your dog? Do you feel you know enough to know the difference?
- You are vulnerable to the whims of the consultant because he now has your dog. If the consultant choose not to ignore your attempts to contact him or her, there is little you can do, outside of going there physically. If he or she is using methods you don’t agree to, you will not know. If your dog comes back with a problem, you will not know how it happened. This can cause guilt and anguish for having sent them away in the first place.
- Dogs with fear aggression, territorial aggression or dominant aggression towards other dogs can react differently with certain people. A fear aggressive dog needs to know he can rely on his owner. If you don’t know how to react confidently when some situations arise, your dog can sense this and lose confidence in you, and may resort to his aggressive behaviors. A territorial or protective dog may regard his owner as his territory which is not something you can work on if he is somewhere else with someone else.
- Dog aggression can be traumatic and it can take a certain amount of time for owners of aggressive dogs to trust their dogs again. It is not enough to take the word of a consultant that your dog is fine, when you have learned to be fearful or cautious. You need to work through the treatment as well as your dog. Trust takes time. In undergoing the rehabilitation with him and witnessing his progress, you can learn to trust your dog be seeing with your own eyes whether he is trustworthy. Otherwise, fear can crop up no matter how you try to disguise it causing you dog to lose confidence in you, or to sense there is a problem and resort to other behaviors.
- Dogs will learn with associations. On your dogs return he may resort to aggressive behaviors simply because that is a habit of how he is around you, or his home. Where as if you are part of the rehabilitation process, his habits will change with you.
- When a dog has undergone a certain amount of treatment, you need to maintain it. Learning how to do this often takes a shift in attitudes as well as practicing new behavior. As much as it takes time for your dog to learn (or unlearn), so it will be for you, too. Any step backwards, or inconsistency can be detrimental. Its better to make your mistakes as the dog is learning. Once the dog has learned with another but then experiences your weak points, the treatment can begin to unravel, and it can be more difficult to get back on track because the consistency is not there.
 “He has to learn to be subservient to you,, in your home, not to a stranger in a distant kennel” p. 116 The Dog’s Mind – Understanding Your Dog’s behavior, Bruce Fogel, D.V.M., M.R.C.V.S. Howell Book house, New York 1990