I agree that not everyone is comfortable writing a negative review. Same here.
However, as others have said, there are ways to do so tactfully.
Even if you don't write a review, what I think is just as important, is that if there is a thread about said SP, make some comment about your negative, or not so great experience. Like:
- she wasn't as gook looking, as I expected
- she did not live up the PSE service level, as others seem to have experienced
- she is not quite the hard body as a 20 y/o
- etc
Others will appreciate it, even if you did not do a review of the SP, in the first place. Of course, do not use this technique to be a coward, and bash the SP.