In objection to sola Scriptura, a Catholic apologist says, "By what authority do you justify your interpretation?" (or words to that effect). Catholic apologists routinely frame the issue in terms of authority. Unless your interpretation is authoritative, it's just fallible private opinion.
It's striking how many Catholics find that gambit persuasive. But that's why they're Catholic.
i) Appeals to authority are used to settle disputes. But for that very reason, an argument from authority can't settle a dispute in the case of competing authorities. If the legitimacy of the authority source is the very issue in dispute, it is viciously circular for one side to appeal to his authority source to trump the opposing side.
Rather, he must first present an argument for the legitimacy of his authority source. He can't deploy an argument from authority to justify the authority source he's appealing to.
The dispute between Catholics and Protestants is in part a dispute over legitimate authority. You have two competing claimants: Scripture alone or the Roman Magisterium. It's premature and question-begging at that stage of the argument for the Catholic to mount an argument from authority based on the Magisterium, for that has yet to be established.
ii) Moreover, by attacking unaided reason, a Catholic apologist disarms himself from arguing for his authority source. His objection generates an infinite regress. If you always need some authority source to warrant your beliefs, then by what authority do you belief in the Magisterium? The Catholic objection just pushes the demand back a step, creating a dilemma for the Catholic apologist. By what authority does he trust in his authority source? What authorizes the Magisterium?
If it's illicit in principle to argue for your position by using unaided reason, then a Catholic apologist has preemptively invalidated any arguments for the Magisterium. If he makes a case for the Magisterium, that's just his fallible private opinion. There's no referee to say which side is right. Unwittingly, Catholic apologists who takes this approach neutralize Catholic apologetics. They can never get started.
Given the Magisterium, he can appeal to the authority of the Magisterium, yet he needs a preliminary argument independent of the Magisterium to legitimate the Magisterium in the first place. But by his skepticism and relativism concerning unaided reason, he forfeits the ability to give a Protestant compelling reasons to believe in the Magisterium. His apologetic strategy is self-defeating.
It's funny how many Catholic apologists are blind to the quandary they've made for themselves. They locked themselves in a cage and thrown away the key.