I would only say this -- sticking to facts, avoiding emotionally-charged or conclusive words (e.g. "dishonest", "evil"), and -- perhaps extreme cases -- not nam

I would only say this -- sticking to facts, avoiding emotionally-charged or conclusive words (e.g. "dishonest", "evil"), and -- perhaps extreme cases -- not naming names makes the claim of disparagement much harder to stick. For example, consider the following: I bought a widget from an online retailer. It arrived broken. It took two emails and three calls to get an RMA. Once received, it took three weeks to receive a refund. Versus Online retailer X is evil.