Any anonymous system is open to abuse, or at the very least to distortion of the truth. People who didn't like a company who left can say whatever they like and are free to stretch the truth, and likewise companies can write their own exaggeratedly positive reviews.
If you want to know what a company is like... go look for yourself?
I have two counter-examples to this from my own experience.
The first is when I was asked to do interviews for my replacement after I handed my notice in at one company. Everyone knew that I was leaving because I hated it, I didn't even have another job lined up. But I was the person on my team _least_ likely to give a bad impression of the company.
The second is when I was interviewing for a startup tech lead role. One of the engineers gave me a tech test, then we had a chat, and I asked, "What's the worst thing about the company?" She told me: "Basically the CEO has no idea how software development works, or agile development, and we have loads of arguments and it's really frustrating. We're trying to hire a tech lead to shield us from that." Needless to say, I didn't take the job...
If you want to know what a company is like... go look for yourself?