Airgapped - hackers can't reach it over the internet, or even via a hacked computer or phone.
Stateless - my seed & wallet get wiped out every time the device shuts down or reboots. If the device gets stolen, there's nothing on it.
This is all irrelevant if the Kendryte K210 chip on your device is hard-coded with a back door to leak your private key subliminally in a signed transaction.