Spectra/Dyneema (UHMWPE) fibers are the strongest fibers available for weaving fabrics, but the puncture resistance of a woven fabric depends heavily on the weave.
The major puncture risks for rafts or inflatable sleeping pads are needle-like (thorns, pine needles, wood splinters, etc.). Sharp-edged rocks could not easily puncture a raft or sleeping pad (unless it is obsidian and it has a razor edge). Thorns, pine needles, and wood splinters can easily poke between the threads of loosely woven fabrics, so the tightness of the weave, not the tensile strength of the fibers, is the most important consideration for a puncture-proof grounsheet material. The most puncture resistant fabrics have very high thread counts (ends per inch) and they are laminated to a heavy film.
Spectra/dyneema fabrics for composite layups almost never have thread counts (ends per inch) greater than 35. Without a heavy film laminated to one side, a thorn could easily slip through this without having to break any fibers.
Tightly woven nylon or polyester fabric with a heavy urethane coating, on the other hand, would bend and break a thorn and not be penetrated. And the urethane coated nylon or polyester would be far cheaper, besides.