Many of these devices promise the earth. I've yet to find one that has and I've tested plenty, through work. I've found cockroaches inside the housing, I've seen mice sat on top of them and I've seen rats gnawing on the corners of them.
If you find one that fulfils what it promises let me know because you'll make me a fortune.
I reckon your best bet is setting traps. Repellents don't work. Poisons will work but you risk the ferrets eating the dead carcasses should one of your targets keel over. As your ferrets are much smaller than dogs, and far more likely to eat dead mice/rats, the