Off market seller found, $3.5M in revenue. No accounting system.

Thoughts on a $3.5M firm being run on spreadsheets rather than a formal accounting system? I'd be doing a QoE but curious how common this is for off market businesses. Previous experience has all be through intermediary.
The QoE would align bank statements to invoices and taxes, but feels like things would be much easier to hide in a spreadsheet accounting system. Thoughts?