Visqueen/generic plastic is often made from low grade resins and recycled materials. Besides lacking the protective qualities that many industry experts recommend (low permeance, longevity, and durability), most generic plastic films do not comply with ASTM E 1745 and are not suitable as long-term below slab protection. These generic products can also vary dramatically from roll to roll. You only have one chance to place proper vapor protection below your slab-on-grade building, so why gamble with products that are not engineered for long-term protection?