You probably have the better alternative to ceiling fans for those who want to spend the money.
You brought up a couple other excellent points in that the hose must be treated like drain pipe by being supported and sloped so no water can collect inside but rather drain.
The other was that most people run them with the bathroom door closed and the only make up air that can be drawn into the room is a small gap under the door. The rule of physics being that to blow air at a certain cfm it must receive air at about the same cfm to displace the blown air. With a small gap, more power is needed to move the rated amount of air through a 4" duct so therefore they just spin without creating enough pressure to move their rated cfm. I don't doubt that your percentages are correct.
Of course another reason they are not used is that I know some wives who don't want any draft period while taking a steam bath.
I wonder if we can write a few more chapters before we finish the book on ceiling fans?