Counter party risk is a significant concern.
Most ETFs are exposed to the ability for another party to deliver on demand, such ETFs operate on a literal promissory note to deliver a specified amount of Precious metals, unfortunately the system is based on a fractional reserve system that would default in the event of a huge demand well exceeding physical supply.
If you wish to keep your Precious metals in a Bank's safety deposit box you must read the fine print to what the Bank's terms of services are. Many only only rent out the box but not guarantee the contents you place in it, and may restrict your access by Governmental decree.
Therefore, you have to be also familiar with what your governmental policy and plan is should there be an economic crisis that can prompt an authoritarian seizure of your financial assets and initiate a bail-in protocol.

In Canada, the Minister of Finance made into law in 2016 a 'Bail in' policy that will freeze cash, investment accounts and other bank held assets for the sake of bank solvency and economic stability.

It can be days, weeks or months before you have ATM access to your devalued cash and in limited quantities.
They may issue bank shares following confiscation of my funds like they did in Greece and Crete.

So, do your homework.