Ann's book is solid on the facts---it's just the "tone" that certain people don't like. Ann's just too talented a satirist for them---makes them embarrassed before their liberal friends at cocktail parties. Besides, everyone knows anyone---let alone a young, good-looking neophyte like Ann---declaring "Joe McCarthy was right!" to the bien pensants in America---both liberal and "conservative"---might as well be standing up in the middle of a Baptist convention and shouting "Cursed be Jesus!"
I just finished "Treason" and read Horowitz' and Collins' book last winter. I really think the reason Horowitz and the Dorothy babe (whose last name I can't remember, but I read her review on FR as well as Horowitz') have objections to her book is style.
I have a kind of unusual background - ultra liberal parents and upbringing, east coast ivory tower atmosphere, parents with all kinds of degrees from pretigious east coast universities. (I left when I was 16, moved to west coast, never graduated high school - turned conservative and am the blackest of black sheep). One quality that the liberals I have seen (closer than I would like) all have is SNOBBERY. Unbelievable arrogance and snobbery. Also they do NOT have a sense of humor - they especially cannot laugh at themselves. They don't like Ann's book because it's raucus, funny, uses exaggeration for humor and to make a point. She uses vernacular that people without university education can understand. It's too low class. And coming from an east coast, highly educated lawyer, a Connecticut blue blood!! It's too much for them.
Horowitz and Dorothy aren't liberals. But they are invested in the highly educated elite myth and Ann is too coarse for them. And since Horowitz used to be a liberal, he still has that aroma, I don't know about Dorothy.