As Physiatrists, I think we need to be careful about the misconception that a certification in Neuromuscular Medicine is a certification in EMG, which it was never designed to be.
The Neurologists' sub-specialty certification for EMG is the Neurophys certification. For Physiatrists, your ACGME EMG certification is your primary PM&R board certificate. The ABEM certification is can look nice on your resume, though I have applied to a few EMG services (who contract with private insurance/work-comp) who preferred ABEM certification.
The Neuromuscular Medicine certification is for Physiatrist or Neurologists who want to manage the complete care of these types of patients. To grandfather in as a Physiatrist, you would almost need to work part time in an MDA clinic for a couple of years.
As it stands, there are no PM&R Neuromuscular Medicine fellowships. In contrast there are 16 Neurology Neuromuscular Medicine fellowships vs. 90 Neurology Neurophys fellowships, with the Neuromuscular requirements including a substantial amount of clinic with possibly little additional electrodiagnostic training.
Personally, I don't see it being all that useful for the vast majority of practicing Physiatrists, especially those in MSK/sports/spine/pain.