I don’t have one and have never used one but having spent years digging fighting positions (Army) my concerns would be condensation and humidity.
I would certainly want whatever I put in one of these in a vapor proof (specialized gun storage bag) bag and some sort of silica material added to the container.My other concern would be once you bury one of these it would be possible to forget its exact location and you have no way to ensure that someone else doesn’t find it. In some states or localities it could be illegal to use for weapons storage...
For myself if I felt I needed an option like this then I would probably rent very small storage unit and put a solid but less expensive safe in it that was near whatever location I felt it needed to be. That way its under lock and key and the environment is controlled and you’re not likely to forget where it is.