Personally, I don't think a shop becoming "inconvenient" is a valid reason to cancel. That is, if you mean you found something else you wanted to do more, or you filled up your calendar after accepting the shop, etc. If you or a family member gets seriously ill, your car breaks down, your house burns down, that's another matter. That would fall under "impossible to do." I had to cancel a shop last month because my car was in the shop and wouldn't be ready in time. The scheduler was not happy, but I did give her 48 hours' notice, and my rating/ability to self assign wasn't affected. I'm sure if that happens again, I'm toast with them!
As Lisa said, it sounds as if you cancel a lot. If guidelines are misleading or don't disclose things that are critical, I think cancellation is valid. But I wonder what you mean by guidelines being greedy? If that means that the overall scope of the project goes well beyond your understanding of it (based on what information was available when you accepted the job), that's pretty subjective. I've cancelled one shop in the years I've been shopping because the job went well beyond the "blurb" provided about it. There have been other jobs that I've realized, after the fact, would be more work than the pay was worth, but I did them anyway (as I'm sure many or most of us have done!) because I either value the relationship I have with the MSC or because I was new with the MSC and wanted to establish a good relationship with it. Now, I probably wouldn't have much compunction about cancelling such a shop.
I don't mean to assume, but it sounds as if you perhaps don't take mystery shopping seriously or look at it as a job. Even though we're independent contractors, we still have an obligation to complete our assignments--unless there is an unusual or compelling reason not to.
When you're in business for yourself, it's really important to fulfill obligations to your clients, meet deadlines, etc. That is, if you want to be successful and get and maintain a good reputation.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.