They're not as difficult as some people say, however they present a slightly higher challenge for the uninitiated for a few reasons ;
-Saltwater fish tend to cost more
-Saltwater fish are a bit less forgiving on mistakes, so they require more regular monitoring
-A saltwater aquarium requires a bit more equipment and products.
So for a first fish tank, it is suggested to go for the simpler solution of a freshwater setup.
After you've become familiar with your aquarium, you can start a saltwater tank with little difficulty.