They work great!!!!
Of course, you would have lots of costly batteries. It takes lots of power to overcome the voltage drop in miles of conductor. You are talking about an expensive system. Since I am not a electrical engineer, the type of system required to build a fence capable of delivering lethal protection is up for speculation. And it would be a gamble of a lot of money that could be easily defeated.
After all of that expense, I would come along, clamp an extension cord to the fence conductor, go down about 10 feet and clamp the other end of the extension cord to the fence conductor. Then, I would cut out about 10 feet of fence conductor between the clamps with my insulated wire cutter. No one would know the fence electrical preventative was defeated. And the fence would be an easy access point for border crossers.