Don't consider teachers with a lot of bad reviews. At the same time, 1-2 negative reviews against the background of a large number of positive ones are not terrible. Any teacher can have a situation where the interaction with the student or his parents is not set for some reason.
Reviews on the websites of companies that select tutors are usually real-most companies value their reputation. But if the tutor has his own website or account in social networks, there will by definition only be good reviews, which may not reflect the real situation.
From the reviews, you can understand what the teacher specializes in. If you need to prepare for the exam, and most of the feedback is from students who have closed the session, this is probably not your option, even if all the reviews are positive.