"neutrally, truthfully, and informatively" is very subjective. I don't believe it will give you any insights. The only thing that can give you some sort of vague idea of the workplace is check the Linkedin profiles of ex-employees, how long they stayed there (doesn't work for new startups...) ? Even better if they are part of some sort of meetup, you can casually meet them and chat, but that's borderline stalking :)

I am on the hiring committee at a university mathematics department. As such, I read a bunch of rec letters. All of them are positive and praise the applicants. But, some of them say things like "Candidate X ingeniously solved Problem Y which I, along with Famous People A, B, and C, had no idea how to do." And others just say "Candidate X is diligent, cheerful, and a hard worker."

In short, +1 to the parent comment. It should definitely be possible to gain useful information from an e-mail that focuses on the positives.