I don't actually know. Hiding feedback for an escort seems like a good way to stop getting any business and I wouldn't see a client who had hidden feedback unless there was a very good reason for it. I can't think of any off the top of my head though!
I think the option to have hidden feedback is a worry for escorts and clients.
Even though AW feedback can be fluffy as hell, it's still serves a purpose. For clients it can show that an escort sees a variety of blokes who think she is vfm, is worth forking out for, and isn't a scam. Escorts get some peace of mind from viewing clients feedback knowing that the guy is genuine, has seen other well-known girls, and is not a danger.
Maybe if it was multiple malicious feedback? I don't know, I had one knobhead leave me feedback only telling people to avoid and it was lies but he has zero feedback himself and all the rest of mine is positive so I would assume guys would look at the full picture of the feedback.
I had a malicious comment posted as feedback by a well known AW troll. I contacted AW and they removed it as I hadn't accepted a booking or even replied to the guy. After searching on UKP I realized he was known to many and has a history of doing this to girls all over the country.
Whilst everyone has the right to post what they like, if someone genuinely didn't enjoy their booking with me I'd prefer them to do a review instead of a couple of sentences on AW.