For most people, chicken on the grill means wings, drumsticks, thighs, or breast pieces that are laid down on the grates and cooked over direct heat. Chicken pieces are a lot easier to deal with, and they all cook at the same time so there is less chance of overcooked meat. (For food safety, you want to cook poultry until the juices run clear and no longer bloody.)
Doing a whole chicken, however, is a whole different animal. The bird has such an odd shape that doesn’t cook evenly. Grill the bird long enough to fully cook the dark meat, and you risk having dried out white meat. You have to find a way to cook the chicken until it’s done, while keeping both the white and dark meat moist.