I think it's fine to write reviews of therapists and psychiatrists on Yelp. There are plenty on there and also on Google. The business owner does not not need to list themselves on the site, so you can write a review of anyone you want. Yelp does have moderators however, so reviews that are negative to the point of appearing spiteful may not approved and won't be shown. There are a wide variety of reviews so it doesn't appear to be biased. I would imagine some therapists wouldn't be thrilled about clients writing reviews and do mask it as a concern for confidentiality. If a client chooses to post publicly then that's the their choice. So long as no one else is named, I don't see why that would be a concern to the therapist.