Viscount Norwich has written many history books, and his skill as a writer is evident in all of them.
In this work, he is writing more as a compiler than as a direct researcher: he is very dependent on the works of others for his facts - something he fully acknowledges-, and though he overlays these with his own views and his great narrative skill, that dependence on others shows in this book more than usually. Given the range of material he has had to work through, it would be astonishing if he had time to assess the value of everything he uses, and to check the facts that he passes on. Sad to say, too many factual errors have slipped through for comfort.
The following are chosen just for illustration, rather than with any aim of completeness. Luke is not regarded as the first evangelist by any reputable scholar. Nor is Polycarp "suspected" of writing the Pauline epistles, or any of them. Norwich writes that he can find no church in Perugia dedicated to S. Lorenzo; it's the duomo (cathedral), as thirty seconds with Google will reveal.
Then, he says that Victor II is buried in the Mausoleum of Theodoric at Ravenna. This is an interesting one, because it is repeated over and over as a "fact" by multiple authors, all of whom rely on a single report that his body was laid there to avoid trouble - which may just mean a temporary stay. So one can't totally blame Norwich for repeating it: but none of these scholars have checked what is available about the Mausoleum. If they had, they'd soon find the truth: the Mausoleum of Theodoric is empty, and has been for some hundreds of years. It has got a beautiful porphyry coffin, true; but the absence of a lid means any visitor can see for themselves that the guidebooks are right. It's not clear if anyone has ever been buried there, but there's no-one at home now, nor is there any clue I'm aware of as to where Victor II's body is.
This lack of care is uncharacteristic of Norwich, but it does mean that the book can't be relied on. He's generally fine on the Popes that he writes about at length, but those he skips over he did skip over, so to speak.
For a reliable and very readable book, one can do worse than do as he did, and read
Saints and Sinners: A History of the Popes (Yale Nota Bene) which is one of his sources.