What is the risk if we use the purchase order module e.g. create a purchase order and receipt the purchase order BUT we don't match the suppliers invoice to the purchase order i.e. we process the suppliers invoice as a cost invoice and allocate the cost to a general ledger account?

One risk is that your GRNI account will never decrease. It is cleared during the matching and approval process.

Besides the risk that mrbraun1 mentioned, there are some other risks, for example, you can not close the production orders which have subcontracting purchase orders, etc......

BR.