I haven't found a technical whitepaper on it but supposedly NAND/NOR flash goes void immediately when reaching a certain high temperature below the melting point of copper. Something about putting a blow torch to the chips for a brief moment...
Hammer sounds easier to verify the destruction worked.
Startup idea: Industrial portable just drill-holes-to-you-device-if-needed & then just-add-water data-erasure bags (single-use, safe to throw away). Would save lots of equipment weight.