I had literally just the day before cashed out. Saw an offer for Nutella ice cream that was essentially free (a little better, in fact, since it was on sale). Submitted a receipt, got the points within a few hours, and redeemed a $25 Paypal that also processed in a few hours. Then the next day, poof, Shopkick was gone. In the end, I only lost about $5 worth of points, so not too bad.
Yes, the cautionary tale, don’t let your credits build up too high at any of the apps/sites, because indeed, they can vaporize without warning.
I recall what’s probably the biggest fiasco of all time, Cyberrebate. I bought some stuff there and got my rebates back and managed to avoid disaster there, too, before they blew up. They ended up basically being a pyramid/ponzi scheme. I remember reading about the lawsuits; some creditors were in the hole six figures!