Cooking in a slow cooker, often makes meat more tender & juicy. This is especially true for unpopular cuts with connective tissue & lots of fat. The long hours in a Crockpot help break that all down. Since boneless skinless chicken breasts are lean and have no connective tissue, cooking for many hours is not necessary. If your chicken is tough, dry, or rubbery, it was cooked for too long. It also may have not had enough liquid & BBQ sauce in the CrockPot.