Two simple critiques: 1) Relative to a financial statement (i.e. budget), it is difficult to predict and measure inflation. This difficulty lowers the likelihood of government fiscal responsibility and increases the likelihood of devestating inflation.
2) A basic second-order effect: What happens when people realize transfer medium is being debased? They'll obviously try not to store value in that transfer medium. How will volatile inflation affect the use of the transfer medium? Why wouldn't users switch to a better store of value, like every single developing market that has ever lost control of its currency?
I am embarrassed for profession of economics that MMT is being entertained as a viable "new-old idea." Of course the government can debase the numéraire of the economy. Obviously, doing so transfers value from non-government to the government similar to taxation. The financial statement of the economy has to balance. This is merely a backdoor way of transferring assets in a deeply unaccountable way.