1) For the metals ETFs I know about, the underlying is held by a bank which is a separate legal vehicle to the ETF. Since the ETF and the bank are both publicly-listed firms, their full-reserve commitments would be regularly audited to a high standard.
2) Brokers participate in risky activity that is unrelated to digital asset trading. This seems like a higher risk profile than a firm that is streamlined to maintain a single ETF.
3) The broker acts as custodian for your digital assets, but not for your shares.
Imagine that you buy a digital asset through your broker and they become insolvent a week later. Assuming your contract gives you no special rights, you would sit on the books as one creditor alongside unpaid vendors and unpaid employees. Years later, you may get back cents in the dollar.
Whereas you would have full and immediate rights to any ETF you had bought through that broker on the same day. They have no influence once it has settled.