Ultrasonic cat repellers

I have lost all patience with my efforts to encourage wildlife in my garden simply resulting in the local domestic cats treating it as a combination of hunting preserve and toilet.

I have looked in to various means of keeping them out. Some plants seem to discourage them, but without some fairly intensive planting of a limited range of species this seems to have only limited success. Lion droppings seem to be an expensive solution. 

What is the situation with respect to ultrasonic repellers? I understand they work, but they also seem to repel species I am perfectly happy to have in the garden. Many manufacturers seem to treat this as a positive selling point. I contacted the manufacturer of one of them who gave me his "personal assurance", but with no evidence, that his device (sharing an emission range with other products whose manufacturers treat repelling foxes as a positive thing) did not affect other wildlife. Given that many species share an audible range with domestic cats I am dubious.

Some expert advice would be appreciated, please.