Debt vs. assets vs. valuation
A standard argument why most "mainstream" Economics treats the level of debt in an economy as irrelevant is that debt is collateralized by assets, so if debtors fail to pay their debts there is just a transfer of wealth between debtors and creditors, and overall there is no change. So for example each debt is collateralized by an IOU, or by a separate asset, and if it is collateralized by an an IOU in effect a failure to pay the debt is the same as a transfer of the IOU from the creditor to the debtor, or if the debt is collateralized by another asset, a transfer of that asset from the debtor to the creditor.
That is both precisely right and entirely ridiculous, because debts are not collateralized by assets but by the valuation (in the sense of sale price) of assets, and while debts are written for a fixed valuation, the valuation of assets can be very variable.
So the IOU of someone that cannot pay the related debt has a valuation which is a fraction of its face value, and the property that collateralizes a mortgage can acquire a valuation that is a fraction of the face value of the mortgage. Even worse, the repayments on a loan stop often at the time where the valuation of its collateral is low.
Therefore the inability to repay debts does not merely transfer wealth, but it can also destroy wealth, in the sense that the reduced valuation of an asset reduces its purchasing power. Therefore balance sheet issues do matter to macroeconomic insights, ultimately because newly issued debt is in effect newly issued purchasing power in almost every situation, not a transfer of purchasing power.