With ETFs, you don't need to be concerned with reserves. I can buy the ETF, go to the market-makers, and ask them to exchange the ETF for either bitcoin, or a cash equivalent. If they can't deliver, they're in trouble.

There's a two way arb in ETFs which keeps the price tracking accurate: - if the ETF trades higher than bitcoin on crypto exchanges: the market makers can simply buy bitcoin on exchange, sell the ETF on traditional exchanges: do this enough and the price difference narrows. - if the ETF trades lower than bitcoin on crypto exchanges: I can just buy the ETF, ask to redeem to bitcoin and pocket the difference. Sooner or later this price discrepancy goes away.