The downside or cork is their weight and cost. The plus side nothing rides as well or looks better IN THE WATER to waterfowl than cork and one can get away with fewer decoys in most situations.
A dozen good cork will outdraw 2-3 dozen plastic. If you hunt someplace with plastic and someone else is using cork and knows how to waterfowl , you 'll get some shooting in.......after the person using cork picks up and leaves.