Don’t get me wrong-if someone is determined enough, strong enough and weak-minded enough (like removing grout on tile manually-you need a weak mind and a strong back) it can be done.
If someone goes out in the desert with a truck with a generator/gas powered device and wants to use something like that it can be done.
But as people who think about it can see, it is harder than just walking across an imaginary line in the sand.
They did an engineering study that showed a battery powered sawzall with the right blade would cut through it in 15 to 20 minutes, or half that time if two saws are used at once. The result is an opening that a single adult can pass through. But like you say, that is a HUGE improvement over an imaginary line or a rickety fence. There are any number of sensors that could easily detect a saw working on a metal structure. I have no doubt that even those few minutes of lead time makes catching them *far* more likely, which in itself is a deterrent.