I don't have pics of any cell core failures. Chief plumbing inspector of my county tells me that a local plumbing contractor installed some town homes (2-story) and used cell core for the W/C risers. After occupants were in homes for a while, they noticed stains under upstairs bathrooms. Long story short, it was when the upstairs toilets were flushed, the water was wicking out of the cell core and forming droplets that eventually leaked down onto ceiling below. Perhaps this isn't often, but once is enough.
Hire a licensed master plumber.