Hello all, I have done 100s of offerwall challenges for kongregate, there is power in numbers so I have recieved over a 1000$ in gems on the Animation Throwdown card game. There will always be a low risk you don't get your reward and the best way to ensure you get a reward is to have a receipt from google play showing you bought credits for the challenge you completed, then you have to deal with the support of the said game challenge, they have the power to send certificate of completion. Dealing with knongregate can get you the points but they will not let them go easily, it has taken some people over 3 request to get payment(power in numbers).
With that being said, I love the offerwall because it saves me alot $$$ on gems and I get to have fun playing new games. There will be challenges you don't get rewarded for once in a while though.