Actually, AC units will perform better in dry environments because the AC unit isn't expending thermal energy in converting gaseous water to liquid water
Swamp coolers work on the exact opposite principle, They convert liquid water to water vapor. When they do this, thermal energy is extracted from the air and stored in the water vapor thus cooling down the air. i.e. water evaporating will cool the surrounding air.
The reasons swamp coolers work so well in high altitude deserts;
1) Thinner air allows the water to evaporate easier.
2) Dry air (low relative humidity) can more easily evaporate liquid water.
The big advantage of a swamp cooler is you only need to run a blower motor and a small water[pump.
There is no need to run the big compressor thats used in a AC unit.
Another advantage of a swamp cooler is that it adds a little moisture to the air, making it a little more comfortable to breath.
2001 Coachmen Mirada (Ford F53 6.8L V10) - Toad 2001 Civic
It won't do MACH 2, but I can get a sandwich and take a pee.