Wendy Doniger (b. 1940) is a University of Chicago Indologist who is despised by many Hindus. Her books rightfully expose the variety of Hinduisms and how many of them are intentionally sanitized versions which are very different from the earlier saucy texts like the Mahabharata and others.
In India, where presently a Hindu-centric government rules, many Indians protest non-Hindus writing about Hinduism and the government often responds by banning these books. Doniger argues well against such silly protesters in her article in The Times of India (March 15, 2015).
Believers and non-believers alike have biases. But certainly one need not be a member of a group to write incisively on that group. Believers can accurately criticize atheists, and non-Muslims can give us excellent insights into Islamic groups. But in the end, we must question insiders and outsiders alike.
I am several orders of magnitude away from being as informed on Hinduism as Doniger, but even I feel free to write on Hinduism — on a subject of which I am relatively ignorant.
Let the reader beware!
Note: Some books by Wendy Doniger