I have never felt that reviewers only want positive comments. I haven't noticed a difference in my ratings regardless of whether my reports are positive or negative..... and over the years I've written a lot of positive and negative reports. BUT, having said that, I have always felt that, when negative comments are made, everyone is holding their breath saying "uh-oh," and negative comments must be carefully explained. Positive reports are easy. The shopper knows, and the editor knows, that the client's management will be pleased and likely no one from the client group will question the report. A negative report will raise questions in the minds of the client's administrative management and they will certainly ask questions of the facility manager and the location's staff. A negative report is much harder to write and needs to be more detailed. My negative reports are longer and more detailed.
Think of how you feel when you get a performance review at your job. If your boss says "Great work, nothing can be improved. I'm proud to have you as a team member," you probably won't ask a lot of questions or want to know why he feels that way, right? OK. Let's say he rates you low in a couple of areas. Most people would question that, wanting to understand why so they can improve. And, if they think the boss has not rated them fairly, they might argue. A negative performance rating needs to explain exactly why it is rated the way it is.
Many clients use mystery shops to determine bonuses, raises, and even promotions, so getting a good mystery shop report is very important to the facility manager and to the employees involved. Your report means they may not get a raise or a bonus, so of course they are going to ask detailed questions, and, if enough detail is not provided, they may well argue with the information provided. Your objective is to provide so much detail that it's clear that you saw what you saw and heard what you heard. You need to provide a lot more detail when writing a bad report. Describe the employees, what they said, what they did, where they stood, and, if you overhead any portion of their social conversation, describe it. Lots of detail. You are being paid for your observations, so these are not your opinions or impressions, they are what you saw and heard.
The managers and store employees will hear what you write about them. Be honest about shortcomings, but give them credit for anything they did right, no matter how small it might seem. I've had some really bad situations during shops, but I've never done a shop that was ALL bad. Point out the good, too. Try to include something they did right in with what they did wrong. Like: Although she did not greet me and offer the special of the day, Susie smiled and made eye contact. Or: Although the cashier and bagger carried on a personal conversation about their plans for the weekend while ringing up and bagging my order, both smiled, made eye contact, and thanked me warmly, inviting me to return.