I can't speak to Mao: The Untold Story, as I've not read it (and honestly find biographies endlessly boring and tedious), other than to recommend checking out Was Mao Really a Monster: The Academic Response to Chang and Halliday’s "Mao: The Unknown Story" as a supplement to her book. The book in question has faced some serious criticism in academic circles, which is why I'm recommending the supplementary text to understand how academics have responded to the book.

That being said, I'm of the viewpoint that all books are valuable, seemingly objective or not, at least for the purposes of historiography (and said viewpoint's influence within a larger society -- academy, popular, whatever -- but that's my research tendencies speaking).

Her memoir Wild Swans is basically as useful as all other memoirs are useful for studying an event: namely, as testimonies that need to be supplemented and independently verified with secondary source material and other primary source documents. (I'd also argue that they're important in order to study the effects of viewpoints, nostalgia, and the like on cultural memory, but I'm not sure if there's research on how English-language memoirs of the Cultural Revolution have affected how the public remembers said events -- although that's what my PhD topic is going to be, if I get into graduate school, so no sniping!)

Mind, I'm sure that all of this is familiar to you, since you're flaired in historiography yourself. ;)

I would take anything she writes with a huge boatload of salt. Her book Mao: The Untold Story has been panned by every single academic because of blatant distortion of sources. Here is an example. Original quote from Mao:

In this kind of situation, I think if we do [all these things simultaneously] half of China’s population unquestionably will die; and if it’s not a half, it’ll be a third or ten percent, a death toll of 50 million. When people died in Guangxi [in 1955-Joseph Ball], wasn’t Chen Manyuan dismissed? If with a death toll of 50 million, you didn’t lose your jobs, I at least should lose mine; [whether I would lose my] head would be open to question. Anhui wants to do so many things, it’s quite all right to do a lot, but make it a principle to have no deaths. 190 billion cubic meters of earth and stone is an awful lot; discuss it - if you insist on moving [that many], I can't do anything about it; but if people die [as a result], you cannot cut off my head. Last year must be surpassed by a bit, moving [say] 60 to 70 billion [cubic meters], but one doesn't want an excessive amount ... As to 40 million tons of steel, do we really need that much? Are we able to produce [that much]? How many people do we have to mobilize? Could it lead to deaths? Even though you say that it is necessary to work on the basic points, how many months do you need to get that done? Hebei says half a year, which will have to include iron-smelting, coal, transport, steel rolling, and so forth; this has to be talked over. – Talks at the Wuchang Conference, 21-23 November 1958. This transcript of the discussion at the top-level conference includes a frank admission of mistakes by Mao and some critical interjections by Party officials present.

As it appeared in Jung's book:

Nor was Mao just thinking about a war situation. On 21 November 1958, talking to his inner circle about the labor-intensive projects like waterworks and making “steel,” and tacitly, almost casually, assuming a context where peasants had too little to eat and were being worked to exhaustion, Mao said: “Working like this, with all these projects, half of China may will have to die. If not half, one-third, or one-tenth—50 million—die.” Aware that these remarks might sound too shocking, he tried to shirk his own responsibility. “Fifty million deaths,” he went on, “I could be fired, and I might even lose my head… but if you insist, I’ll just have to let you do it, and you can’t blame me when people die.”

Her work on Mao is definitely not valuable. There are many reviews done by academics which point out the problem with her sources.