Termite barriers are designed to prevent undetected entry of termites, and, by and large, all the barrier systems out there work. However, when failure occurs it's almost always due to an installation failure. This can be caused in two ways.
1) The installer installs the barrier incorrectly.
2) Barrier is damaged after installation.
So always try to ensure that barrier systems are installed correctly initially, and if other trades have to work after the barrier is installed, make sure they understand how it works and what their role is if they must handle it in any way during their particular work on the building site.