A review can only be left by somebody who had an appointment, after that appointment has happened. There is no way to post a review from the outside, which is what makes the number worth reading at all.
A studio's rating stays hidden until it has at least five reviews. An average of one is not an average, and showing it would tell you about one person's afternoon rather than about the studio. So a studio with no rating is almost always a new studio, or a busy one whose clients do not write reviews — not a bad one.
Ratings and badges cannot be bought, and a studio cannot delete a review for being unflattering. They can reply to it, and a considered reply to a poor review often tells you more about a business than the review did.
Read the recent ones and read the middling ones. The three-star reviews are where the specifics live, and the specifics are what let you judge whether the problem would be a problem for you.