I found it interesting, and he makes some claims that seem bold or that are contrary to (what I understand to be) conventional wisdom.
-The central thesis of his book is that balance-of-power diplomacy keeps the peace far more effectively than the rule/morality/internationalism-based ideas that gained traction after the horrors of WWI.
-That the "balance of power"-type system (defined as one where several states are forced to interact and compete with one another and none is strong enough to impose their will on the others) is historically very rare, having only been seen in Europe and Warring States era China. He instead says that empire-tributary was the principal type of relationship between entities throughout human history.
-That WWI was principally caused by German belligerence after the death of Bismark and secondarily by Britain refusing to send a clear enough signal of opposition to German aggression before it was too late.
-That WWII was almost entirely the work of Hitler's personality and refusal to wait for Germany to naturally grow into the dominant European state.
-That the Western Allies completely bungled the opportunity to counter the Soviets after WWII. Firstly, by not advancing further east and secondly by stubbornly insisting on democracy and self-determination in Eastern Europe, instead of conceding that they were in the Soviet sphere of influence. This supposedly drove the Soviets to take a harder line on controlling Eastern Europe as opposed to settling for a Finlandization. There is also a general theme of Stalin running circles around the democracies when it came to foreign policy throughout his rule.
-That 20th century American statesmen consistently failed to articulate a national interest to work towards in foreign policy and instead pursued the moral or principled course to a fault. As an example, he says that America made a mistake in coming out so strongly against the British and French during the Suez Crisis and against the Europeans in general on colonial issues.
-That the Vietnam Was was was a mistake, caused by American leaders vastly overestimating the amount of leadership the Soviets had over the world Communist movement and by their fear of a united Communist state in Southeast Asia. Kissinger maintains that American leaders were ignorant of or underestimated the degree of the Sino-Soviet split and that China never would have tolerated a united Southeast Asia.
The fundamental thing to keep in mind is that Henry Kissinger's Diplomacy is not a history book. It is an international relations book born out of the realist school of international relations theory. While history and international relations are related fields, practitioners in the two disciplines have different methodologies and goals.
History informs many of the inferences that international relations theorists make. However while historians are typically very concerned with evaluating and reinterpreting the significance of facts and details, international relations theorists are instead looking to take those facts and details in order to extrapolate a broad universal meaning. This means that it is very difficult, or even impossible, to answer your questions from a historical perspective: Kissinger is not looking to write about history. He is defending an international relations theory. While historians also have their biases, historians seldom extrapolate views that they hold to be broad universal truths (although I suppose you can always make broad conclusions on human nature, but I digress). Historians are much more likely to qualify that their assertions are only valid insofar as they relate to context of the evidence they have examined. So what is indisputable in the history of the Mediterranean is not necessarily applicable when examining China or India: that is to say, while scholars ask questions of the causes and qualities of the fall of the Roman Empire and rise of the Frankish Kingdom, these causes and qualities cannot be applied to the Sixteen Kingdoms period of Chinese history. To their credit, international relations theorists are typically very careful about precise wording, and often qualify their conclusions with many caveats so that they are aligned with the evidence they present. However, as you have seen yourself in Kissinger's book oftentimes their interpretations are informed by these broad universal truths that historians instead try to avoid
So unlike a history book, all of the claims Kissinger makes makes are, in one way or another, tied to an underlying theory. That theory is that "great powers" should respect each other's sphere of influence. This ties into the idea that a bipolar world, that is to say one with two great powers, is the best world order for international stability (and it follows that a multipolar world, with many great powers, and a unipolar world, with just one great power, is instead an unstable system). This sort of "Balance of Power" argument has had a multitude of iterations and forms in international relations theory, all with varying degrees of acceptance from the academic community. While today there are a many competing, more nuanced theories, during the Cold War ideas on "Balance of Power" reigned supreme in both academia and the US State Department. Backers of the theory range from the enlightenment thinker David Hume who regarded it as a scientific law (although I suppose he was co-opted by the realists, having died long before the Cold War), to Kenneth Waltz who while at Columbia and Berkley argued that “If there is any distinctively political theory of international politics, balance of power is it,” and Robert Jervis (also at Columbia) who claimed that “balance of power is the best known, and perhaps the best, theory in international politics.”
Jack Levy at Rutgers and William Thompson at Indiana University in Bloomington wrote a paper in 2005 titled "Hegemonic Threats and Great Power Balancing in Europe, 1495-1999" where they summarize and go on to test the key ideas on balance of power that doubtlessly inform the interpretation of history brought forth by Kissinger and Kissingerites. You might be interested in looking it up, as their goal is to test the balance of power theory against history. In their paper, they also explain that while there are many variations to the balance-of-power theory (and disagreement about its key concepts and what they purport to explain, not to mention the serious challenges leveled at the theory since the fall of the Soviet Union) nearly all balance-of-power theories start from the proposition that states will coalesce to balance against threats of hegemony over the international system. The paper also explains how proponents of the theory will almost always share the realist assumptions that the key actors in the international system are states (and territorially based actors that can act as states) and that these states will act rationally in order to maximize security and power in an international system that is anarchic. The paper could be a useful tool through which to re-assess Kissinger's claims.
Opponents of the "Balance-of-Power" argument include some other realists who instead believe polarity and hegemony don't exist outside of theory and that only relative capability of states measured against neighbors and rivals (with "relative capability" intended as an aggregate of political, economic, and of course military factors) instead determines what actions are taken by states and thus the main implication for stability (Charles Doran at Johns Hopkins is a major proponent of this idea). But there are also scholars who throw realism out the window: the Liberal school of international relations theory, which traces its roots to the enlightenment and gained steam in the 1980s to eventually become the dominant school of thought following the collapse of the Soviet Union, instead believes that a peaceful international system is not so much a consequence of selfish states trapped in a balance of power as it is the natural result of economic cooperation which renders the disruption brought about by war far too costly. Liberals credit U.S.-led institutions with fostering interconnectivity through capitalistic economic principles as well as the exportation of democratic ideas. Although it is a claim disputed by some Realists, many Liberals also hold that the checks on power intrinsic to democratic states do not allow them to go to war with each other.
There is indeed there is little in Realist theory to disprove the Liberal idea that international institutions, both formal and informal, not only create a regulated space removing elements of uncertainty and doubt from interactions between states, but also resolve problems of information asymmetry, foster interconnectivity, and once put in place are very difficult to destroy. We have seen voluntary adherence to international institutions and their constraining norms, which seems to disprove the Realist assertion that states are selfish actors interested only in gaining power. It would seem that the predictability of outcomes, stability, and order generated by international institutions outweigh their constraints.
Other schools of thought within International Relations that would certainly disagree with Kissinger included Marxism and Constructivism. Again, Kissinger's is writing a book that lays out the theoretical underpinnings of the decisions he made while in government (or, equally likely, to rehabilitate those decisions). The points of contention will naturally be numerous.