Does anyone have a mystery shop company they don’t like?

I haven't come across one yet.

Overall, it's not wise to bash MSC's. Schedulers work for multiple companies and grouching about one can get you into trouble with others. There are two about which a note of caution would be appropriate:

Red Quanta: They are struggling to pay folks. I don't know the inner story, but they are based in India, there are language barriers and questions about the criminal history of one of their schedulers.

Cirrus Marketing: They require a separate invoice that you have to submit separately. If you don't, you will not get paid. They also have a pay schedule that makes you wait close to four months before being paid, so it's really easy to lose track of your work for them unless you are organized.

Waiting 4 months isn't too bad if you have other shops,and the one you are waiting for you don't have to buy anything.