In the comments section at The Conversation, Professor Richard Holden does mention one fairly common criticism of MMT
Accounting Identities as Behavioural Relationships
I have addressed this in a previous Critic Engagement piece.
Bill Mitchell has two blogs that addresses this and Professor Holden should be happy that it includes some modelling.
The confusion lies in those who want to represent MMT by the sectoral balances framework – as if that is the extent of it. In this blog – Stock-flow consistent macro models – I explain why accounting is important for a macroeconomist.
Flows add to stocks and you have to track those relationships consistently over time. One still has to theorise about what determines the flows and their inter-relationships but if you try to make statements that do not add up then you are lost in ad hocery and nonsense.
The two linked blog posts above should help anyone with any queries about human behaviour and the Stock Flow Consistency of the Sectoral Balances and Accounting Identity.
At the end of the day, MMT plugs in behavioural assumptions to this accounting identity. These assumptions are contestable but consistent with relationships that hold at the macro level.