Polyester resin has the following disadvantages: 
# Strong styrene odour
# More difficult to mix than other resins, such as a two-part epoxy
# The toxic nature of its fumes, and especially of its catalyst, MEKP, pose a safety risk if proper protection isn't used
# Not appropriate for bonding many substrates
# The finished cure is most likely weaker than an equal amount of an epoxy resin