Effective operation of the border requires that the government enforces the law regardless of what detection and deterrent methods are used to a great extent.
Given a little time, fences can be cut, vehicle barriers removed or destroyed, ditches filled in, etc.
I will agree that it is less obvious that the laws aren't being enforced if the illegal crossers don't have to leave a trail of destroyed barriers, and that destroying the barriers requires extra effort and tools.
However, barriers and surveillance are really tools to aid in enforcement. They do little by themselves.
Preventing migration using sturdy fence does not require that you stop people from crossing. It only requires that you inspect the fence frequently and keep it in good repair.