The problem is one of architectonics.
A pre-publication story about Reid's taking up the story quotes him as saying “The war is 85 percent of the story, and that what a lot of people are waiting for.” And that is exactly the problem.
For Manchester, two thirds of the work--Visions of Glory: 1874-1932 and Alone: 1932-1940 took place before World War II. In those volumes, Manchester slowly, painstakingly delineates the growth of Churchill's character--how a shy, sensitive boy lumbered with a disapproving, erratic father and a remote, glamorous siren of a mother forged himself into an apparently extroverted man of action. (A transformation very like that made by the young Theodore Roosevelt). In particular, Manchester excelled at evoking the painful relationship of Winston to the father who often harshly berated him, and whose champion he longed to be, long after his political star burned out. Likewise, Churchill's loving relationship with his wife Clementine--their playfulness with each other, her own insecurities and dashes away from his shadow, and their children--the "kittens" as he sometimes called them--are central to volume 1, and are not lost in volume 2.
By contrast, the nearly 6 years of World War II take up 930 of Defender of the Realm's 1,053 pages of text. Churchill's second premiership (1951-1955) is covered in a mere 41 (pages 996 to 1035), and the last ten years of his life in an even skimpier 16 pages (1037-1053). It's simply not enough. Martin Gilbert took this same period--from V-E day to Churchill's death--and wrote Never Despair, (1988), which I find to be the most compelling volume of his biography, generally less readable than Manchester's. In Defender of the Realm, while some human touches persist, the brevity obscures the extent to which Churchill found, or failed to find, resolution in those last years. And it's not because the material isn't there, or doesn't relate to the themes of the first two volumes.
Let me take one concrete example. In Gilbert's Never Despair, he publishes (at pp. 365-372) the full text of Churchill's essay "The Dream" (the text is online here) which tells the story of a 1947 dream he had, in which, painting in his studio at Chartwell, copying a torn portrait of his father, he "suddenly felt an odd sensation," turns around with his palette in his hand, and sees his father, sitting in his red leather armchair, "evidently in the best of tempers" and looking as he did in his prime. The conversation which ensues between them is one in which Winston, who never discusses his career beyond his serving as a Major in the Yeomanry, tells his father about all of the changes in the world since his death in 1895. He does so without in any way mentioning his role in them. After Churchill describes the World Wars, the Cold War and the Atomic Bomb to his father, Lord Randolph (who has referenced his low opinion of his son's capacity, while casually saying "But of course you were very young, and I loved you dearly") says:
"Winston, you have told me a terrible tale. I would never have believed that such things could happen. I am glad I did not live to see them. As I listened to you unfolding these fearful facts you seemed to know a great deal about them. I never expected that you would develop so far and so fully. Of course you are too old to think of such things, but when I hear you talk I really wonder you didn't go into politics. You might have done a lot to help. You might even have made a name for yourself."(Gilbert, Never Despair at 372)
He gave me a benignant smile. He then took the match to light his cigarette and struck it. There was a tiny flash. He vanished. The chair was empty. The illusion had passed. I rubbed my brush again in my paint, and turned to finish the moustache. But so vivid had my fancy been that I felt too tired to go on. Also my cigar had gone out, and the ash had fallen among all the paints.
"The Dream" isn't just an amusing fancy; it's Churchill's final reconciliation with his father's memory, and his quietly besting the man he loved with so little return. Lord Randolph sees his son's abilities, and admits that he loved him "dearly," as well as blaming himself for being too harsh to Winston. Winston knows that he in fact has done all of the things that his father is depicted as wishing he had done--he has succeeded beyond Lord Randolph's wildest dreams for himself, let alone for the son he always underestimated. As in life, he cannot tell his father that--but he realizes that his father would wish that success for him and can savor the irony at the end. He puts down the brush, too tired to copy Lord Randolph's portrait, and having left his own traces all over the palette.
There are many such nuggets in Gilbert's last volume, and in other materials regarding the Churchills and Winston's last years. The saga of Churchill's last ministry, and his battle with fluctuating health is told both by Gilbert and in the diaries of his doctor, Lord Moran (disapprovingly referenced, and his testimony rejected in Defender of the Realm, but so briefly that the attempted refutation reads more like a refusal to see the Great Man's weakness, his "Black Dog" than a reasoned position). The travails of his children are only fleetingly mentioned, even though they form a large part of the denouement of this long saga. Clementine's fierce protectiveness of her aging husband, and her destruction of Graham Sutherland's portrait of him, a gift from the House of Commons for his 80th birthday, is fleetingly mentioned, with no sense of the controversial nature of her decision to destroy such a gift, and of her own mildly favorable reaction to the portrait before discovering how much her husband hated it. The welter of war buries the personalities that we have followed for two volumes, good reading though it is.
In short, Churchill's own words "jaw-jaw is always better than war-war" could have been helpfully remembered in this readable and interesting book, one which misses being a masterpiece by losing Churchill's story in that of his times.