Short Answer:

Some reviewers had serious complaints, but any commentary on a China related topic has to be read with an eye to the complexity, unresolved historical issues, limited access to documents and contemporary political sensitivity of the subject. Jung Chang's Wild Swans is a more successful work, being explicitly a family memoir, rather than the more difficult target of a history of Mao.

Discussion:

To learn what "historians think" about a particular book, you can always check reviews in historical journals. In this case, because Mao is on that edge of history and political science you want to check a bit more broadly into that latter discipline too.

For example, Lucian Pye -- pretty much top of the "China watching" game in the US at the time-- reviewed it in Foreign Affairs in 2005, a review that's decidedly less enthusiastic in places

The husband-and wife team of Chang and Halliday have nothing good to say about Mao, whom they depict as a cruel tyrant who did not even believe in the ideology he imposed on China; they make no effort to explain how so many people, both Chinese and foreigners, fell under the spell of him and his myth.They have done a major job of researching various archives and interviewing several hundred people, and Halliday's command of the Russian language led him to material that depicts Stalin as having had a bigger hand in the founding of the Chinese Communist Party than previously thought. But they tell a story that is not too different from what is found in standard histories of the period-except for their unrelenting personal criticism of Mao. Thus, for example, while most histories treat "factional struggles" as standard procedure in Chinese party politics, Chang and Halliday present them as the work of the evil tyrant Mao. In the end, they may exaggerate his faults, but their work will surely succeed in tarnishing the legacy of the Great Helmsman.

For a longer and more critical review, see Albert Chan's lengthy article in Pacific Affairs

One would expect special insights on the nuances of behaviour and power relations from Chang, a Chinese scholar and former Red Guard who experienced the Maoist era first-hand. A subject as complex as Mao's life and times cannot be treated in black and white terms. Yet the authors take things at face value, offering up page after page of wild assertions, leaving readers no room for reflection or imagination. No sensible person would want to apologize for Mao, especially when official censorship and cover-ups have prevented many Chinese from telling their stories. The tremendous policy blunders and human costs of Mao's policies should continue to be revealed and discussed, but by trampling all over their sources in their efforts to provide a simplistic and sensational analysis, Chang and Halliday have botched their chance.How could such an angel of death, who brought about so much suffering and destruction, still maintain the respect of so many of his people even 30 years after his death? Was Mao's charismatic ability to move millions based simply on terror, cruelty and oppression? Mao: The Unknown Story contributes little to the resolution of these pressing questions

In any event the Chang and Halliday book is now 15 years old, based on research that is older; there's been a lot more access to a lot more material since then.

Chan himself has a 2018 volume "Mao's Crusade: Politics and Policy Implementation in China's Great Leap Forward" -- much more up to date, albeit much more technical and narrow in its focus. Reviewers of this volume have complaints too

But even today, your take home message is that with so much Chinese Communist Party material still secret, historians and political scientists still struggle to understand just what happened under Mao. As Neil Diamant, reviewing Chan's book says

If Mao was indeed so powerful, why was there variation in individual and provincial responses to his programs? Was Mao that much more skilled (or Chinese officials that much more docile and politically unsavvy) than Hitler, who also led his country to disaster but unlike Mao was nearly killed in an assassination attempt by top officials?

To which the answer is "we don't know". There are a range of models for what happened in China under Mao, with scholars arguing the different positions all with some evidence, but we don't really have enough to come down definitively on one side or the other.

The thing that makes "Mao - the unknown story” particularly valuable to me is that Chang Jung was herself the daughter of Communist Party officials, and a Red Guard in her youth. They were "Communist Party elites"-- but then her father criticized Mao, and with that turn came a world of misery for her family. So it's hard to see this book wholly as a dispassionate history-- because Halliday is a historian and this book is deeply researched, it may appear this way, but it can also be seen as a bit of intellectual autobiography. Chang doesn't present a "nuanced" view of Mao perhaps because there was nothing "nuanced" about what happened to her family.

You can argue with the scholarship in places-- and obviously some do-- but what you have in this book is an account by someone who lived in the system, writing together with an accomplished historian (her husband). People often misunderstand their own systems, particularly totalitarian and secretive ones-- but there's an intrinsic authenticity to "this is what a thoughtful woman who grew up in this system came to believe about it"; even if the analysis can be faulted, the authenticity can't. Halliday has a particular strength in that he's got expertise in both Asian and larger communist world politics, particularly the rhetoric of communist leaders -- not too many China watchers have edited the memoirs of Albania's Enver Hoxha.

So it's a worthwhile book, written by people with valuable expertise. It shouldn't be the only thing you read on the subject, if you want a balanced view. And even with that "balanced view" -- there will still be big gaps of information, questions whose answers might lie in still locked Chinese Communist Party archives. As noted at the top, Jung Chang also wrote a family memoir called Wild Swans which has many of the virtues of "Mao- the unknown history" without some of its faults.

Sources:
Chan, Alfred L. “Mao: A Super Monster? Review Article.” Pacific Affairs, vol. 79, no. 1, 2006, pp. 97–103.

Pye, Lucian W. Foreign Affairs, vol. 84, no. 6, 2005, pp. 153–154.

Diamant, Neil J. The China Quarterly, no. 169, 2002, pp. 206–208.

Wilson Center, Researching the History of the People’s Republic of China