It's not the end of the world to do accounts in a spreadsheet. In fact for certain sorts of things, they are really ideal.
However, it is a huge mistake to keep your general ledger or book of original entry in Excel. The reason has nothing to do with making mistakes or errors and every thing to do about fraud. Paper is the gold standard in anti-fraud accounting systems, particularly if you follow the standard rules like ensuring books are kept in pen. Paper to Excel is fine. Replacing the paper with Excel is not.
However, it is a huge mistake to keep your general ledger or book of original entry in Excel. The reason has nothing to do with making mistakes or errors and every thing to do about fraud. Paper is the gold standard in anti-fraud accounting systems, particularly if you follow the standard rules like ensuring books are kept in pen. Paper to Excel is fine. Replacing the paper with Excel is not.