I believe the claim with the electromagnetic (rather than ultrasonic) ones is that instead of sound they use the magnetic field that is intrinsically generated by wires carrying current, and the device uses the wiring in the house to send pulses around that generate changing magnetic fields that upset the pests. The theory is that the device is then effective pretty much anywhere in the property that has mains wiring.
To be honest I'd be a bit sceptical as to what other devices this will affect within your home - if it's messing with the electrics enough to create a changing magnetic field of a magnitude that affects animals I'd wonder about what this does to other electronics in your house, as some things are more sensitive than others to the quality of electrical supply. I could certainly see it potentially causing e.g. hifi systems to make some odd noises!
Depends a bit on exactly how it works though, and also on the circuit used for the mains wiring system. The nature of the power supply transformers in other electrical items you own will also be a factor in this, as there are various ways to design and build these and some are more robust than others.
I'm also doubtful as to whether the strength of field generated is actually useful. Animals are certainly potentially sensitive to high field strengths, but they don't seem that bothered by the relatively weak fields that are present in all mains electrics and I don't really see how this device is going to generate a larger field.
I can't give a definitive answer and I don't know if they will actually be effective against rodents but I'm very sceptical, and finding proper independent peer-reviewed data on them seems to be near impossible (which is so often a sign that someone is selling snake oil...). I did find this - not the most recent source though: https://digitalcommo...16&context=vpc8
The other factor to consider is perhaps: If they work that well, why aren't lots of professional pest control companies using them? If I can buy one for £10 on Amazon and it actually has a statistically measurable and useful contribution to reducing site pest count then why don't all of our pest control contractors fit them as standard as part of our service contracts? 
