Advantages - much easier to clean up (shiny impervious plastic surface versus fragile netting); better surface for adhesive to stick to, so a very solid hold; mimics human skin very well, so looks incredibly realistic when seen from above, especially partings and crown swirls.
Disadvantages - non-breathable, so sweat gets trapped under the base; any unglued areas of the base make a "plastic bag" noise when touched; risky to use strong tape on the thinner poly bases, because you could rip the base when trying to remove the tape; the base can be shiny and visible at the hairline, especially in strong downward light - there is a distinct straight-line edge to the piece.