I don't think you can assume the average person is honest/transparent. You can't assume the person you're negotiating your salary with is going to tell you if your offer is much too low. You can't assume HR isn't going to lie about the reason the fired somebody. You can't assume the company will give you any heads up if they're thinking about firing you.

A lot of people offering "feedback," especially public, especially if it's negative, are just angry venters. And this goes both directions E.g. Glassdoor.

Not saying it's good to have a chip on your shoulder about any of this, but it definitely shouldn't be swept under the rug.