This maybe gets said a lot (to different degrees somewhat erroneously) by post-Keynesians...

The neoclassical synthesis? Sorry, doesn't really have a leg to stand on and demonstrates a complete disregard for heterodox literature - if not the entire discipline of economics... https://twitter.com/chrisedmond/status/1295145859240226817

Shall we @chrisedmond take just one key concept - crowding out effect? Absent fall in aggregate demand (i.e. bad times), in normal times public investment is believed to crowd out public investment. This view is underpinned by theory of loanable funds and limited source of funds.

A rising fiscal deficit places increased demand [gov must compete with private sector for a scarce] amount of aggregate (national) savings, which in turn forces to clear the loanable funds market at higher rate. (Image below from Mankiw economics textbook h/t @StephanieKelton)

On the supply side, increased government borrowing reduces the amount of savings and loanable funds, which further increases rates. Since in loanable funds model, investment = savings, a lower amount of savings mean less private sector investment.

As the deficit increases – higher interest rates are offered to in exchange for further borrowing or as a premium against the risk of default. In sum, the higher interest rate crowds out or chokes off private sector investment.

I don't know where to start on the PK and MMT critique - whats new- and sorry if I wont cover it all (b/c there are number of axioms that can also be attached to crowding out theories - such as ricardian equivalence).

As Lerner noted, money is a creature of the state - government with free floating exchange rate that issue debt denominated in their own currency - so state is not financially constrained by availability of loanable funds.

Moreover, the availability of private sector credit is not constrained by loanable funds - credit is created endogenously by banking system, which entails non-neutral effects for the real economy.

In an endogenous monetary system, crowding out depends on spending and saving/investment decisions of other agents and sectors of the economy - to what extent economy is at full capacity.

Below capacity, crowding out through the operation of monetary policy would only occur through deliberate discretionary action of the central bank, not from markets. The interest rate is under the direct control of the monetary authority.

A budget deficit increases the net financial assets of the private sector – instead of decreasing the level of national savings. In fact, instead of putting upward pressure on interest rates, deficits place downward pressure on short-terms rates. (h/t Mosler, Wray, Lavoie)

Kalecki argued that public sector investment increase the profits of the private sector – so the reverse causality can be considered crowding in effects, by reverse writing his equation: Profits net of taxes = Consumption out of profits + Investment + Budget deficits (h/t Lavoie)

Even if resources/inputs are fully employed, provided that the budget deficit is relatively in proportion w/ heightened productive capacity of economy, inflation does not have to be an outcome - deficit raises growth + production. (h/t Mitchell)

Deficits might be needed to help crowd in private investment; rather than crowd out, budget deficits might be a quasi-permanent feature needed to maintain aggregate demand, depending on certain saving decisions of different agents in the non-goverment sector. h/t Arrestis+Sawyer

Sustained fiscal consolidation - due to fears of crowding out - could encourage households and firms to take on higher levels of debt and sell of assets, which can in turn lead to economic and financial instability. h/t Goldey and Lavoie

Indeed, sustained fiscal consolidation due to fears of crowding out - can lead to significant build of risks and vulnerabilities elsewhere in society and economy. Think environmental breakdown, degrading infrastructure, weak welfare systems etc.

This is not exhaustive but hopefully provides a beginning of an answer to your question. End/