The one time I posted a negative review it was promptly taken down after the host complained. They then proceeded to submit a negative review of me (a personal attack really) that had no basis in fact and I had already provided Airbnb with proof earlier in order to get my money back. It was a huge PITA to take that review down. Grew very disillusioned with Airbnb after that, they really don't provide any protections against malicious hosts.