From my engineering / business degrees, I rely on Return On Investment (ROI) knowledge to to make these decisions. I CAN'T make the numbers work for route shopping; taking into account the planning, the vehicle mileage, maintenance, and low shop fees / reimbursements. I CAN make shops work where I normally drive by on a periodic basis and where I might use a particular product or service.
For me, mystery shopping isn't a real profitable endeavor. It is a cost avoidance activity like using digital coupons. I'm in it for the low effort reward, not earned income.
I guess it's like marriage. There's an economy of scale for two people living in the same household along with benefits. Of course, like selecting mystery shops, not selecting an appropriate housemate can lead to a costly separation / divorce.
It pays to select and plan accordingly!