OK. It's hard to comprehend something you are unsympathetic to. And there is always the risk that in trying to comprehend something one might become sympathetic to it. So on that I agree.
But I refer you back to Karl Keating's 'Catholicism and Fundamentalism' for a detailing of some of the errors in fact and historical detail that Boettner should have had a handle on if he was going to write a book on the subject. If Boettner got all of that wrong did he really have a handle on doctrine either except to say he disagreed?
I have Keating's book. It's a good read. I would have to look at it again & compare it with Boettner's own words. I think it's possible to blunder in historical matters, while making substantive theological points.
It may seem odd to defend Boettner, but:
1. I'm a contrarian.
2. I think Boettner often gets a tough time from Catholics, not always fairly.
3. I believe sympathetic understanding of those with whom one heartily disagrees is very important.
4. As a convert, I remember how bizarre Catholicism seemed to me, looking in from the outside. So I suspect it is very difficult, even for an irenically-disposed Calvinist, to "get into the mind" of Catholicism. In Boettner's case, there was the additional difficulty that he was (if the last chapter of his book on Predestination is anything to go by) a patriotic American as well as a conservative Calvinist - and the CC is unavoidably foreign to the US & its culture. There is a point beyond which the CC cannot be Americanised - the Pope is the living symbol of this. And, of course, it is hierarchical, not democratic, in its structure. So are some other Churches in the US - but they are not historically the enemy of Protestantism.
5. Boettner makes what seem to be good points at times - even if some of them can with equal truthfulness be made against Protestantism. But I think that mutual recrimination has nothing to be said for it.
I think he assumes, too often, that Catholics cannot be intelligent, thoughtful, sincere, well-educated - and also, be orthodox and believing Catholics. Granted, there is great deal of stuff in Catholic life that from any Christian POV needs to be corrected or uprooted - but one does not judge a religion (or any institution) fairly, if one looks only at the sewage. His book on Catholicism would have been much stronger, if he had judged it by its best examples. Catholicism has produced some appalling human beings: but is that because they were admirable examples of Catholic life, practice, and sincere interior religion - or because they were anything but those things ? An adequate critique of Catholicism cannot judge it by its tyrants and Mafiosi alone - it has to take account of people like St Francis of Assisi, St Philip Neri, or Cardinal Cesare Baronio, as well.
Likewise, other religions should be judged from those people who best exemplify what those religions stand for - not by those people who make no effort to exemplify what they stand for.
Catholics of the kind who are waved around as examples of the horrors of Catholicism, and people like that, show what Catholics are, if so disposed, capable of sinking to. They are examples of the depths Catholics can sink to - not of the heights they are meant, and can by God's grace be enabled, to aspire to.