You study the effects of your process with regard to meeting the specifications. If the DFMEA process has been properly done, if you meet the specifications, the concerns of the DFMEA have been addressed, whether you know specifically what they are or not. This is one reason that I always stress that the PFMEA should address the supplier's process, and its potential failure modes with regard to the specifications.
Going back to how I understand FMEA came about - it was to prioritize actions on the things that have most potential for harm (at NASA) so if you take out the Severity category you loses a lot of the power of the tool.