When you use a set of lowering coils you are basically just installing a shorter coil which has the tendency to flatten out the control arms some. Installing a set of lowering control arms provides the factory coils with a pocket that is set down farther allowing the truck's front suspension to site lower, once again flattening out the control arms.
When you combine the two of them together you can end up with the control arms that are actually higher at the ball joint side than on the frame side. This will make getting a wheel alignment very difficult (or impossible) and also ruins the geometry function of a control arm front suspension (Which is actually called a short/long arm control arm system) The factory design adjusts the camber angle during cornering to provide the most tire contact with the road. When you flatten the control arms out too much you take away that ability which can create unstable driving in some situations.