The risk that IMF would be concerned about is that the value of their holdings could go to 0 overnight by a wallet hack, data loss, an unscrupulous government employee, or the world running out of greater fools.
Also thinking, since it's extremely difficult to regulate, was wondering if some very clever people with huge holdings have formed a cartel and are doing a lot of price manipulation to prevent it from fully collapsing (maybe even during a rise in price, causing it to drop early to prevent a major run up). But that's just a theory.