That is correct. No method apart from the binding ritual has been successful in making contact; and apart from specific cases like with Amon, the ritual never fails to conjure them forth... Now, as to whether the pact goes as planned is another matter. The stronger their Ego, the harder it is to bind and suppress them. If they are too overbearing, their appearance and personality will have more sway over you... But that is mostly a minor inconvenience.