Maybe Protestants are "deficient" because the majority of them were founded in heresy.
What is or is not heresy is, of course, a debatable point. It could conceivably be argued that the Roman Church is not the "One Holy Catholic and Apostolic Church," since the See of Rome has ceased to be in communion with the patriarchal Sees of Constantinople, Jerusalem, Antioch, and Alexandria. It could also be argued that the assertion that certain doctrines endorsed by the Roman Church, such as the filioque, are heretical.
