One day I'd like to build a fence to keep my cat(s) in yet still go out, but until then I just remembered these devices for dogs and wondered if they could be used for cats
I don't think they are a good idea for several reasons.
First of all the way they work is by either shocking the animal or scaring with a high pitched noise, which is quite cruel. Secondly, animals have to be taught where the fence boundary is. You might be able to train a dog not to go past a certain point, but this is much more difficult with cats.
Also, the fence is designed to stop your pet going outside, but it won't stop other animals coming in. Your cat will still be vulnerable to predators.
Finally a shock or high pitched noise might scare/hurt your cat, but there is no guarantee it will make them stay within the yard. They might run out, but then they will get shocked when they try to get back in again.
I wouldn't bother with an invisible fence. Cat enclosures and cat proof fencing are more reliable. They can be discreate and even quite nice looking these days