Bitcoin futures trade on the CME, a well known and well regulated US-based exchange. They are cash settled so no bitcoin custody issues. The spot index price source the futures trade in reference to is a well documented methodology sourcing from multiple bitcoin exchange sources.

https://www.cmegroup.com/markets/cryptocurrencies/cme-cf-cry...

https://www.cmegroup.com/markets/cryptocurrencies/cme-cf-cry...

By comparison, with Grayscale, who knows how custody works and what exchange exposure they may have.