I do notice a lot of negative reviews on Glassdoor and consistency in the kinds of major complaints that individual companies receive, so I wouldn't say the system is totally useless. However, Glassdoor is bad at evaluating which reviews should be approved and which ones should be removed. Both my company reviews were removed without them notifying me. My reviews were thoughtful and didn't violate their guidelines. (One of my reviews was lukewarm and the other one was very positive, so it wasn't just a matter of them removing negative reviews due to the company paying them.)
When I asked them why my reviews were removed, they said "Per our Community Guidelines, we only allow members to submit one review, per company worked at, per year. We have removed your reviews due to violation of this guideline." However, I did not violate that guideline.
Hi Elizabeth, we have responded to your review by private message. Please follow up there with any additional questions you may have.