Working with living creatures involves many variables outside the control of a trainer. While some trainers may offer guarantees, I suggest owners read their "fine print". When is the last time your Veterinarian offered you a guarantee?
In lieu of guarantees, I have always offered individual appointments. I believe this is fair to clients, and a more realistic perspective for professional service providers.
NOTE: No amount of training or work sessions by a professional can ever supersede what you teach your dog in everyday life. For this reason, a dog's behavior and conduct is a responsibility that ultimately rests with the owner.