They're not competent enough to make an initial proposal for how much taxes are due, but they are competent enough to doublecheck a manually filled out tax stat

They're not competent enough to make an initial proposal for how much taxes are due, but they are competent enough to doublecheck a manually filled out tax statement and not send you a tax bill which you don't owe them? The blindly trusting will get ripped off regardless of whether returns are prefilled, assuming a government set on tax fraud (an irrational assumption in my opinion.) Prefilled returns are the opening round in a negotiation with the government. In any bargaining situation the opening offer is the weakest party. You should prefer them to make the opening offer, especially if you distrust them. Better that you check their numbers rather than them checking yours.