That biographer and reviewer are at cross purposes is apparent from the review's first paragraph, where the reviewer expresses bafflement at McGrath's distinguishing his approach from that of other CSL biographies. It turns out the reviewer has no knowledge of the biographies in question -- but that surely is not McGrath's fault, given that the books are well-known and widely circulated. More significantly, McGrath's biography is intended to be half of a two-volume set, the other book being devoted to in-depth discussion of Lewis's ideas. Smilde is aware of this -- he refers to the companion volume by title -- but not having it to hand he choses thereafter to treat the second book as non-existent for his purposes. Given that the reviewer constantly chastises McGrath for not having dealt with this issue or that, one suspects many of the discussions he wishes to see are in fact in the second volume specifically devoted to exploring Lewis's major ideas in depth.
After a single paragraph touching on a few things Smilde thinks McGrath got right, the remainder of the review-essay runs through the far more lengthy list of things he thinks McGrath got wrong. These range from including too much about Tolkien (can't agree with him there, obviously) and too little about Barfield (a valid point. but again I'd want to check the companion book), and far too much about Narnia (I cdn't agree more). Elsewhere he dings McGrath for making an accurate statement without citing a specific piece of evidence to back it up. To McGrath's exploration of Lewis's abandonment of poetry and shift to fiction Smilde responds by denying that Lewis ever underwent a 'shift to fiction' (which seems a plain disregard of the evidence). He is particularly indignant that McGrath never refers to George MacDonald; for my own part, I was shocked that he fails to mention David Lindsay.
But what to make of Smilde's criticism of McGrath for not including in his (extensive but selected) bibliography of secondary works on Lewis's life and thought two dissertations, one by Norbert Feinendegen (2008) and the other by Adam Barkman (2009)? The answer lies I think in Smilde's other contribution to this volume, an extensive (sixty-page) essay on what is sometimes called C. S. Lewis's 'argument from desire'. In his abstract to this piece, Smilde states that one of his goals for this essay is "an attempt to make the English-speaking world aware of a major contribution to C. S. Lewis studies published in Germany by Norbert Feinendegen in 2008" (p. 33). This suggests to me that Smilde brings up Feinendegen's and Barkman's names, not because he thinks it likely McGrath would have consulted either, but because they're things Smilde wd have cited had he been writing this book.
In the end, I think Smilde views McGrath's work entirely through the prism of the ideal biography Smilde wd like to read, in which the things that interest him would receive the most attention and topics that don't interest him are scanted. Since the reality (the book McGrath actually wrote) does not correspond to the ideal (the philosophical and theological work Smilde wd have preferred), he finds it "a book of uneven quality, with more low than high points". For my part, I'd say it was a v. good book, albeit w. some shortcomings. While not the definitive Lewis biography we've all been hoping for, it's certainly the best biography of CSL since Green & Hooper. And that's no small achievement.
Finally, since it's always better to read people for yourself than to rely on other people's descriptions of what they wrote, here's a link to an extensive listing of points about McGrath's book that drew Smilde's attention, with Smilde's comments thereon:
I've only skimmed this, but it's quite interesting to see such erudition on display. My favorites among his critiques and comments were
"This book has a total of 257 sentences beginning with Yet"
"This idea does not seem worth committing to paper."