Oof. Frequent use of the passive voice is just a bit of a red flag in my book.

I encountered a similar issue recently as I was reading through a book on Hellenic reconstructionism. The author frequently used the passive voice to (seemingly) stealth his own off-topic opinions into the text; as if his opinions are some sort of vague consensus, rather than just his own opinions. Ofc, this gimmick might seem pretty obvious to anyone who has a basic understanding of English grammar and syntax and is paying attention to what they are reading.

Regarding the dodgy source-attribution you mention, I've seen this a lot when an author is trying to pass off their work as being academic. Throwing in tons of citations does sort of present a veneer of scholarly rigour. Of course, it's easy to see for yourself if the citations actually confirm what the author is trying to claim. I'm guessing a small percentage of readers actually investigate a book's bibliography. Thus, an unscrupulous (or just sloppy) author can make all sort of spurious statements and generally get away with doing such.