The biggest risk is paper bitcoin: ETF issuer sells 1,000,000 bitcoin to the investor, but they only hold 200,000. They will tell the public they only have 200,000 investments, so they can have a signed proof of reserve, and we will never know they actually collected money for 800,000 more. This way, indirectly they inserted 800,000 BTC into the supply. I know government agencies should catch such a trick, but apparently the same is true with gold. There is a lot more paper gold than physical gold. Through custodians and complex accounting the paper bitcoin can be created.