Most of the scams with mystery shopping revolve around you receiving an email when you are not signed up with anybody to do mystery shopping. Many of the scams consist of sending you a substantial check (usually an odd amount like $1495) to deposit, go do some shops at Walmart, Home Depot or other store names you recognize, keep a couple of hundred bucks for your work, and then wire transfer the balance on as your 'final shop' of the series. Ooops, their check was a forgery on something that looked like a legitimate shopping company and it bounces. The checks are convincing enough that they make it all the way through to the account on the other end that disclaims them so their bank must restore the money to their account. Then the bounce travels upstream back to you. This bouncing process may take several weeks.
There are also scams that have folks signing up for more and more and more 'trial offers' in anticipation of 'winning' a $500 gift card or a laptop or even a substantially lower cash payment. They know that no reasonable person will continue signing up after 5-10 so they never legally owe anybody anything, though they have collected for the 'hit' they directed to various websites.
Stick with legitimate companies--sometimes they feel scammy enough on their own but at least they are real.