In a nutshell, this depends on your power goals and application, some stock DME’s are flashable, while others are not and may require a piggyback system. The Pre 1995 (OBD I) cars such as equipped with M42, M60, M50 and M50TU Motors have DME’s that cant just be flashed, those may require a piggyback system such as the Ostrich II. All while as OBD II (1995 and after), Such as M52, M54 etc come with ECU’s that are typically flashable.
For a everyday street car, a lot of people would find a reprogrammed stock DME to be sufficient, however if you are looking for absolute control and power, then you may find its best to go with a standalone ecu system. It is however best to consult your tuner in regards.