Of course yes, this is the best way to save wealth in the olden days, that people don't have access to the bank easily, because they live in remote areas. I have seen when a piece of land was given to a bot by his late father as inheritance. When the boy went to the land to make use of it, during digging, he discovered a sack buried under the ground and when he opened the sack, it was filled with money that his Dad kept for him on the piece of land.

I don't consider it safest anymore, because, whoever knows that you kept your wealth buried underground will easily have access to it when you are not around. He can spy on you, and know the location or use metal detector to know the location.

Not safe at this time, especially cash because fiat depreciates and lose value overtime. So it is better to put the funds on bitcoin since it still gives you the same opportunity to be your own bank, and the value increases overtime to keep the value of your funds.