You were told correctly. They work much better in low humidity.I am told that "evaporative" generally don't operate satisfactorily here because of the high humidity.
For those that are curious: an evaporative cooler has pads all around the outside. A water pump wets the pads. The fan inside then blows the cooled air into the house.
There can be as much as a 25F (14c)difference from outside to inside.