You're right, all solid savings plans must fit comfortably beneath a mattress. Reminds me of the old saying "If I can't lie down and sleep on top of it comfortably, it's not an investment"

Embed it in the floor, since you ask.

Not that I think this is the ne plus ultra of security, but since having a digital wallet doesn't obviate the existence of valuable physical documents (eg passports, title deeds) you might still want to use a safe to protect against fire, burglary, and so on.

One distinct benefit of a physical store is that removal or tampering are more obvious.

If you burn your cash there's zero risk of it being stolen. But what matters is that you have access to it in the future, and I'm a lot more confident of that with a safe than with a complicated piece of electronics that can only be used by connecting it to an even more complicated piece of electronics.

I've had a significant number of USB drives, (and SD drives) get corrupted and simply stop working. This seems to happen with heavy use, but it also happens just sitting in a drawer untouched - for quite a bit of time. I'm not trust these suckers as the sole repository of things as valuable as my pictures, much less cash.

USB drives have the advantage of small. There's literally a hundred places I could hide one in my house. Hell, I could hide 20 fake ones haphazardly and one legit one really well.

The chances of everyone doing this and being comfortable with it is pretty low though. We need brain storage medium.