The biggest problem I see with the use of these is the fact that most of them are in banks.
Harry Browne wrote in one of his books some time back (1970s!) that this could be a very bad thing if for some reason the feds decided that things were getting too out of hand and declared a "bank holiday" to put the brakes on whatever sort of panic was going on. He suggested that if it were desirable to have such a box that one might seek out a safe deposit company, one that wasn't part of a bank.
I've never seen such a place, myself, or any indication that they even exist.
OTOH, there was a bank up the road that closed, due to a merger in the past several years, and that building is now occupied by some lawyers. I'd imagine that the mergers that seem to keep on happening in the business might present the opportunity for some enterprising soul to pick up a building like that for not a whole lot more than commercial real estate costs in general, and hopefull the safe would still be there and such. One would have to deal with staffing, and time lock nonsense, and perhaps an armed guard or two, but it seems like a viable business opportunity to me. You'd just be renting out boxes for people to put their stuff in, no more, so I'm not thinking that any banking laws would apply, though that probably wouldn't stop the "officials" in any given area from trying to apply such, I guess a lawyer would be a part of the startup expenses too.
And it'd be one hell of a sturdy facility, too. I can still remember tv footage that was on right in the aftermath of Katrina where the reporter was standing in what was left of this bank in Gulfport, MS (?), but the safe itself was doing just fine...