You can do a review if you have a confirmed booking as davie mentioned it is in the rules.
For me personally I have had bookings confirmed which have then be cancelled by them for whatever reason but it has been done in good time so have not felt the need to leave a negative review in those cases.
What I would 100% review negatively is if I have a confirmed booking, set off to the meet then it is cancelled as my time would have been wasted, there would have been an expense to travel and I would not have time to find a replacement. Thankfully that has not happened to me, I hope I am not tempting fate.