Walls, nor anything else, as a standalone system, will ever work 100 percent all the time every day forever and ever. But as part of a comprehensive border securement program, walls could be highly effective in securing the border.
Personally, I think instead of utilizing an insane number of border guards 50 miles inside the border at check points, or stationing border guards in SUVs in rural scrub brush, or dragging tractor tires along the side of the roads so they can see footprints in the morning, they should put those guards along the border every couple of hundred feet and secure it. Don't need a wall to do that.