No, that's not it. I'm starting to doubt myself on this one, as I spent a few hours last night searching around and found nothing stating that the durability wasn't good. I remember reading these studies:
Neither says anything about durability. I could have sworn I read something that said that many of the new synapses didn't last - maybe I'm wrong though.
Those pig studies are encouraging though. One states that there was an 11dB gain across all frequency bands, but that it wasn't enough to be clinically significant. However, I'm not so sure, especially if the synapses aid in words-in-noise type hearing (and the fact that OTO-413 is extended release should increase the benefit too).