In most cases I prefer something that is more potentially passive than potentially scalable. Obviously it’s great to have everything, but given the choice I prefer income streams that just work and can be automated so you can do other things. You have to be careful to manage your desire to scale something with your desire to make it passive. Sometimes less is more because less gives you freedom.
What’s the catch? None, really. Cash back apps act as affiliates for many online merchants, which means that whenever you make a purchase through one of the apps, they get a small commission — but then, they give you a portion of that commission as “cash back”. For example, if I buy a pair of Nike shoes through the Ebates app (or website) and spend $75, Ebates may get a $10 commission but then they’ll pass $7 back to me. It’s basically a way to get sale prices on stuff that isn’t on sale!