With all ETFs you need to verify that they have the goods and can secure them and won't be shut down and don't have sinister motivations (such as walking away with the cash). These two guys made their fortune from suing FB. It just seems like you are adding a layer of risk on top of an already risky "asset".
They have ETFs for just about every major currency in the world so it isn't only about the difficulty of obtaining the underlying asset. However, since they're trying to define Bitcoin as a commodity and not as a currency, I think they will be fighting an uphill battle.