In academic circles the late 20th century saw the culmination of the practice where no academic ever criticised the blunderings of any other academic.
Well, that shows Berkley to be an ignoramus with an axe to grind. Let's list some genuine 20th-century historical controversies. These are from WWII and related history, because that's what I know the most about.
- The Functionalism-Intentionalism debate over the origins of the Holocaust and the political structure of Nazi Germany.
- The debate over Hitler's war aims: was he intending to conquer Europe, Eurasia, Eurasia and Africa, or the whole world?
- The Historians' dispute among different political views within historians in West Germany in the late 1980s.
- The New Historians in Israel, who challenge the traditional narrative of the founding of modern Israel.
There are many, many more. Disputing other people's historical theories is one of the primary ways that historians improve their understanding of history.
(This is used in support of Berkley's controversial thesis, that the ancient Egyptians spoke Welsh.)
That's not controversial. It's nonsense. The only way he could support that claim would be if everyone who has learned Ancient Egyptian were part of a conspiracy to conceal its being a Celtic language, part of the Indo-European family. Since Egyptian is an Afro-Asiatic language, that makes his theory less linguistically plausible than the Black Egyptian hypothesis, which is quite a feat.
Since the titles of Grant Berkley's other books are The Discovery of the Ark of the Covenant and The King Arthur Conspiracy, you can safely write him off as a conspiracy theorist and a pseudo-historian.