Additonally you need to look at the criteria. All too often a customer is asking for an 8D as a punitive measure and not for correction. GM is notorious for that. They are quick to put a supplier on CS-1 or 2 and beat them to death with the 8D/Drill Deep/Read Across.
When the 8D is used to brow beat a supplier you will never have a quality event.
The other downfall I see is that the customer will never come out and look at what the 8D is describing. They will read it and reject it without understanding how it applies to your process. When a supplier puts honest effort and time into the 8D process and gets shot down by someone that does not know their process, you as the customer make it a wasted effort. You will not get the same level of response again.
Work with the suppliers, ensure they know what you are looking for in an 8D. Follow up on implementation and if they develop a cost savings based on the 8D, don't demand the same piece price decrease.