In response to the title - no they haven't. On the contrary, they've been very, very good to me lately. I've made 100's of thousands of credits for minimal $ and very little time spent. I never do the offers to buy products (Olay, meals-in-box, etc...), only the downloadable games with in-game purchases. Occasionally I'll do one for a subscription service (ASPCA, Sirius, American Heart Assc., etc...) where you make a monetary donation and then just cancel your subscription within the first 30 days after getting your rewards. These offers are all dependent on your geographic location and what offers you've done previously.
I used to have issues a few years ago, but that was mainly my own fault from me constantly changing my Ad ID and them not being able to associate my account to the offer. Now days most of the games have a popup that shows immediately after you complete the first goal showing whether the offer was attributed to you, so you know whether to continue or not.