Most accounting spreadsheets fail the same way. They work for the person who built them, in the month they were built, and then quietly stop being trustworthy: a range that does not extend to new rows, a hard-coded number pasted over a formula, a reconciliation that "balances" because someone typed the difference into a plug cell.
A good accounting spreadsheet is less about clever formulas than about structure. The data lives in one place, every total can be traced back to the rows ...