What the IRS version leaves out
A substitute return gives you the standard deduction and little else. Business expenses against self employment income are ignored, so a contractor who received forms reporting gross payments is taxed as though every dollar were profit. Cost basis on investment sales is ignored for the same reason. Dependants, filing status, and credits you were entitled to are all commonly missed.
Self employment tax is calculated on that inflated figure too, which is why these balances get large so quickly.

