Humidity around here in summer is usually 45-50%. I have not seen swamp coolers here aside from an auto mechanic garage. They work in very low humidity. Scotty's Castle in Death Valley had one but it was built 100+ years ago before A/C was used much. My cousin in Phoenix says they work during dry heat season up to mid-July but that they are not effective in later monsoon season.
But James may be right. My observation is that 93F/34C is the highest temperature I find comfortable in shade when humidity is very low. And I was in Phoenix in September when I made that observation. I don't Phoenix humidity goes up unless there is active weather.
I still don't understand split system, because Jason references it vs. ducted system which serves an entire house or al least one floor of a house. . Is split system just a different one room setup vs. the typical window unit? Window units are in windows so the exchanged hot air is vented to the outside. If you are only cooling one room, surely a split system is more expensive and if so how much more efficient is it to pay back that difference?