Coins that are put in grannies cookies box are safe. But if they're going to be cash, maybe in less than a week, they're already destroyed.
I am aware of that technique but if you're living in a crowded city and you're just renting an apartment, I'd put that on a bank.
And that will serve as emergency fund but if I already have one, I'd place that on Bitcoin no doubt much better and safer if the people like me knows how to keep it.
Cookie box will rust and that rust will deposit on your coins. Great idea if you want to spend some time cleaning them after you dig them out.
Also, imagine the box rusting all the way through and some of your coins getting lost deep in the dirt.
There are so many ways to store valuables other than putting them in the ground.