[zaarn]

You can protect against reverse voltage, ie, voltage from the ground line.

A simple diode is the easiest way, using various power MOSFETs can reduce the voltage drop induced and you can even build a circuit that allows charging the battery and supplying power on the same connectors while also protecting against reverse voltage.

It's neither hard nor expensive.

[Arnt]

TIL.

Are you saying that it's possible to protect against all kinds of shenanigans on the ground line even if the device has no other ground, or only against negative voltage?

[zaarn]

There is only reverse voltage and over voltage.

Ground is basically your reference, if your ground has potential above your voltage input, you're in reverse voltage, if your ground is too far below your voltage input, then you're in over voltage.

Both can be detected and prevented from causing damage via various means on both sides of the usb plug.

Overvoltage can usually be solved by having a Zener diode within acceptable overvoltage short the thing to ground and blow out a fuse, this will cut the circuit and prevent further damage.

I recommend this video from GreatScott: https://www.youtube.com/watch?v=7Tk5ghH_U2s