Surely, the border fences 'work' at their own personal, narrow level. That is within a radius of one mile perhaps, after that it's not their problem - it's another jurisdiction.
Unfortunately, mankind already has a long history of building walls and History has taught us a few things: walls must be
manned to be effective. As exemplified by the Berlin wall or the
Siegfried Line. This is where it gets expensive.
Because anybody can climb/bypass a wall with proper equipment, even if it is tall and electrified. When they jump onto the other side, cameras and sensors will not stop them. Technology will digitally capture the even of trespassing, but will not intercept a human body in quick motion.
But since a huge chunk (possibly 50%) of illegal immigration actually consists of visa overstayers (people entering the country
legally), it's obvious a wall is not the solution.
Looking for simplistic solutions to complex problems is bound to fail.