To me, the accuracy of the method depends fully on how it's done. If team A comes with evidence, and team B is asked to prove the authenticity of the evidence without communicating with team A, then this method could very well be credible.
Team A could try to forge evidence by embedding a different recorded hum, but then again, team B could detect but various means, like seeing that components of the original sound near the 50 Hz band are gone, or that the phase of overtones from the 50 Hz don't match that of the fundamental.
Forensics is by its nature not an exact science, because the evidence (compared to scientific lab research) might be lacking. You need to consider various evidence against each other and reach a conclusion. There are of course situations where this particular method (ENF) would be useless, such as heavily compressed audio or if a noise gate was used. But that is not to say the method is bogus and never useless. Remember that in this case it was only used to corroborate the authenticity of the recording after it had been put into question by the defence. Who knows whether the judge might have decided the recording was credible even without this evidence.