So as I understand it (from this and other threads):
Sikorsky managed to persuade the FAA that their S92 gearbox, unlike anyone elses, was immune to any failure except 'an extremely remote one'.
There has been a series of incidents (up until now over land), where failures other than an external pipe leak (if this is what the external isolation valve is meant to deal with) have led to emergency landings.
One could conclude that the MRGB is not as brilliantly engineered as Sikorsky believed and that some serious litigation is inbound if it is proved to be the cause of the very sad Newfoundland crash.