My long-winded experience:
In Oregon during the summer we have huge diurnal temperature swings. For example, yesterday it was 82 degrees during
the hottest part of the day, but at the coolest point at night it was 40 degrees. So you would think that a whole house fan
would make perfect sense here - and at times and under specific circumstances it does.
A few years ago I purchased a fan from Centric Air (Centric Air Whole House Fan - Best Whole House Fan | CentricAir).
It works remarkably well, except, as several others have pointed out it draws in a TON of dust and allergens. We live at the
end of the Willamette Valley, the "grass seed capital of the world" and my wife and children have the worst allergy reactions
I've ever seen in humans. So without filtration at all the windows a whole house fan is not usable for us. To get maximum
cooling you have to open (and close during the day) windows in each room, so that means filtration in each of those windows.
I initially tried to use typical furnace type filters (MERV 13) in the window openings, but that did not work well because the fan
is very powerful and unless the filters are fastened securely, the fan will suck them right off. Plus, unless they are perfectly sealed,
the MERV rating essentially becomes zero. I then built wooden frames for the filters and that worked better, but that meant that
every day the windows would have to be closed. It was a chore. I have a cyclonic dust collector with huge expensive filters made by
Wynn (MERV 15 - still not HEPA), so I designed a system with two of those types of filters controlled by a motorized louver that
opened and closed when the fan powered on and off. I placed that system in the crawlspace (our house is on a steep slope, so
part of the crawlspace is enormous - literally 30'x40' 8' feet high on one end and 20' at the down slope end). It was serviceable and
cooled the house significantly, but having only one opening meant that there was a stream of cold air in the stairwells and pockets
of warm air in areas away from the stairwells. That could be mitigated somewhat by circulating air using the main air handler, but was
not optimal. Also, I showed a friend the setup and he immediately asked: "Did you do a radon test?" Yikes! So I did a radon test and
by dumb luck levels were considered acceptably safe by EPA standards. That part of the crawlspace is well ventilated and has a tidy,
well-installed vapor barrier.)
Then starting in the summer of 2018 we experienced extreme forest fires with choking smoke. Visibility at times around our house was
probably on the order of 100 feet. There is no practical way, economically speaking, to filter out smoke. I do not believe the whole house
fan has been powered on since that summer.
) I do remember doing the math after the install and found with the high cost of the fans, I don't think I would ever recover the cost of savings in my lifetime, haha. Think they were close to $400 each I paid. But with all the "Green" push now adays, I am sure a young couple would think it is a great when they buy our house. Funny but the fans only use the solar when the sun is out, on cloudy days they use our power grid. But again they are not On much and they are very quiet...I got sold/took during our roof replacement 
