I have read the book and its criticism. TL;DR: The book suffers from cherry-picking for telling a story. It ought not to be taken seriously as an endeavour in study of Hinduism. I mean, its in the title - An ALTERNATE History. Otherwise it is a fun read. (Imagine a book called - 'Nazism - An alternate history' based on Inglourious Basterds.)

The criticism comes from people who are taking the book seriously. :)