The biggest threat looks to be the methods of laundering that are cropping up -- I'm starting to hear of instances where offline-chits and money transfer agents are accepting old currency without any markdown. So it looks like laundering hasn't received as large a blow as anticipated, and hence the experiment may very well fail by the Dec 30 deadline.
One positive thing to note is that a majority of the citizens are tired of the parallel economy and are openly supporting the government's moves. Hence it should give the Modi government the cover and incentive to add-in more stringent checks and regulations to take on the laundering. This is probably the only hope for any success in this experiment.
BTW, whether this experiment succeeds or fails, it's a net positive in the long run as it has shown: 1) that a government can take steps towards stopping the parallel economy, and 2) do so with the people's support.