I often encounter a popular idea that deconverts from a particular belief community provide a uniquely authoritative insight into that community and its practices and thus a uniquely authoritative basis to reject it. The idea is like this:
“Oh, you want to live in New York? It’s a hellhole, believe me: I lived in Brooklyn for twenty years.”
By the same logic, if you meet a former Catholic-turned-fundamentalist-Christian who now rails against the Catholic Church, well his perspective is especially authoritative because, hey, he was a Catholic. And if you meet a former Baptist preacher-turned-atheist who rails against Christianity, well, his perspective is especially authoritative because, hey, he was a Christian.
However, in my experience, the last people I go to when I want to learn about a particular group is an angry deconvert. While those folks may have legitimate insights, they also typically have a chip on their shoulder the size of Obelix’s menhir.