It seems the two most common reasons for guys to write a bad review on a girl they have seen several times are:
1. They have pressured the girl for services that are not on the menu under the belief that she will agree after he has seen her a half-dozen times, but she has turned them down, so he retaliates with a bad review.
2. Guys, especially newbies, over the course of several sessions lose track of the where the line is between fantasy and reality. They didn't write a review before because they have fallen in love at first sight and want to keep the girl for themselves. At some point, they make a grand gesture (marriage proposal, come live with me, lets start dating, etc.) and the response is something like, "Dude, get a grip. This is just a fantasy service". They get their feelings hurt, get angry over the rebuff to their affections, and write a bad review in retaliation.
I'm sure there are other reasons, but I have discussed this with some providers when they seemed to have only one or two outliers that were bad, with the rest of their reviews being pretty consistent, and this has been the two reasons most often given for the bad ones.
Some veterans, myself included, will often wait to write a review until after the second date, because some providers can be a little shy on the first meeting with a new customer, and you just know the second meeting will be better than the first. If I think she has given me the best experience she is capable of, I will write the review after the first meeting.