Finally, there are a lot of disadvantages for being on a completely centralized digital payment platform from an individual liberty point of view. Imagine that a significant portion of your net worth is just a number stored in a "secure database" which falls under the jurisdiction of some government. Now image that you vocally disagree with the government's policies enough for the government to start taking notice. It becomes very very easy for the government to shut you up - all they have to do is strongarm the bank to freeze that number. Its not as hypothetical as it sounds as the Indian government has done this before.
A large portion of black money was deemed to be held up by the bureaucracy - bribes paid by private individuals and corporations to government employees and politicians. Had demonetization been done along with a hiring freeze (and subsequent trimming down) of the bureaucracy, I'd have been more certain that the motives behind demonetization were to root out corruption. However, the bureaucracy is now larger and more powerful than ever under this government. Thus, the incentives to seek bribes still exits because of this power imbalance between the powerful bureaucrats and relatively week individuals.