Trust. The banksters got that name for good reasons: they're know criminals. There are now two classes of people in the US: banksters, and those who must obey all laws or face jail and/or penalties.
Examples include, but are in no way limited to:
Are you willing to trust people like that to hold your Bitcoins for you? If so, what's the matter with you?
Banksters are the problem. Bitcoin is the solution.
"Your keys, your Bitcoin. Not your keys, not your Bitcoin" — Andreas Antonopoulos
Then, there's the possibility that the fund (ETF) you put your money into is being run by another Bernie Madoff kind of person who will send you quarterly statements, but hasn't actually invested a dime of your money.
Before you go spouting off about regulators and auditors, keep reading:
It has been well known for some time that the government regulation agencies have a "revolving door" with the very entities they're supposed to be regulating. People often leave government to go work for the places they were supposed to be regulating. Sometimes people leave the banks to go regulate their "buddies". Again, are you able and willing to trust them? Not me.
Ask yourself, "Where the hell were the auditors and regulators during Bernie Madoff's caper?"
Trust is just one of many reasons why a wise person should be very cynical about ETFs in general, and about Bitcoin ETFs specifically.