Problem was, as it came out, that MS never actually bought the bullion and thus was charging storage fees on non-existent items. If you went to remove or claim your silver, they just gave you cash as settlement, making the scam opaque.
The cynic in me says GS will do something similar, like put a very high valuation on the FB units they sell, both marking them up a great deal over what they paid FB for it AND inventing some of the units out of whole cloth (i.e. issuing 10,000 units for sale when they only bought enough FB for 1,000 units), then, over time, the FB units will be seen to "decline" in value and GS will pocket the profits twice, first on the markup of the units, then on the decline.