There are a lot of good companies out there. The ones that are not paying in a timely fashion (or at all) include Restaurant Evaluators, Franchise Compliance, Goodwin and Freeman. With the first one, nobody is getting paid, with the second one there is still hope of payment, though it may take legal action. With Goodwin and Freeman it takes following up after the pay cycle is over to get your payment. You will find that shoppers scream loud and long about companies that accepted work and won't pay without chasing them.
On the other hand there are a lot of complaints on forums about jobs being 'unfairly' rejected. I certainly will not point to specific posts/threads where my sense is that a shopper screwed up versus the company being difficult. I have had shoppers send me private messages indicating that they did everything perfectly except _______. If I ran the MSP, that 'except ____' in many cases is so crucial to the shop that I would not be able to accept the shop either.
I'm getting pretty close to 9000 shops. I have had one rejected for something that was provably NOT in the instructions--it was subsequently accepted after a bunch of grief. I have had one rejected for something that I am pretty positive was not in the instructions but my copy of the instructions did not save properly to my computer to prove my case. I have had two shops rejected because the client's locations refused to provide me with the material I needed to have for successful completion of the shop (that really burned me!). But there have been so few shops rejected that they are all quite memorable. The only time I have not been paid for accepted work was when a company closed their doors unexpectedly.
The flip side of this is that every year I collect without issue thousands of dollars in fees and bonuses and thousands of dollars of reimbursements of use to me. So in the greater scheme of things the handful of unpaid shops is not even big enough to consider a mole hill, much less a mountain.
Start with your beginner companies--Market Force blue portal, Bare, Maritz, Corporate Research International--to get your feet wet and then register with other companies as you have time. If you follow the instructions you DO get paid.