I tend to steer clear of anything written by Whelton because I find the standard of his scholarship less than satisfactory. I haven't read the specific work you mentioned, but this review seems to confirm my sentiments:http://www.cjoc.ca/pdf/Vol2-W-3%20Popes&Patriarchs3.PDF
It's funny because I just found that review before you posted it. I was, however, put off by the review itself due to the frequent use of insulting adjectives used by the author of the review. The substance of his review is interesting though, in that sloppy citations and scholarship are often reasons I dislike most modern apologetics work, although I must say Whelton did touch on some issues that many Roman Catholics use as trump cards for proof of the papacy in history and I appreciated a response to these claims being spelled out. Also, I think the reviewer's personal stake (he is an Eastern Catholic, and a doctoral student at a major ecumenical institute) may contribute to the in-my-opinion vitriolic review of a popular work as if it were presenting itself as a scholarly contribution. Hence I wonder how much of what he says of Mr Whelton's work is actually true. I will have to investigate.
I would like to see however a popular treatment of Roman Catholic claims (specifically, the endless lists of patristic quotations, some of which are forgeries, which appear in such books as Ray's "Upon this Rock") that is irenic in style but which addresses the issue comprehensively. Maybe I will reach out to some people and try to collaborate on such an endeavor myself one day. Who knows.