That's my main fear and hesitation (with recommending) them. A simple plug in device, on its own, won't be able to sense a poor earth. It would need a proper test to check the resistance of the earth path to true earth, by an electrician. I am not an electrician so I can't really comment too much, but an RCD would offer a degree of protection (it doesn't need an earth to function - it works by comparing the current difference of the live and neutral lines). And to offer some protection against a poor earth, you could use an isolating transformer + RCD - but really you shouldn't be plugging into and using such poor wiring in the first place.