It's very likely the metal in the electric fence would be stolen and sold for scrap. For the fence that isn't stolen, it just might be shorted out by a tumbleweed somewhere along the border and start a wildfire. Or aren't there any more tumbleweeds between the Gulf and Pacific?
For those unwilling to try to go over the fence, there's always this approach.
For those unwilling to try to go over the fence, there's always this approach.