From what I've hard from Uber drivers:
. It is not possible for them to refuse a ride once passenger has boarded. The only way to do it is to ask the passenger the destination before the ride began.
. The virtual line in airports is designed to prevent this. A driver who gets fixed up with a short ride can return to the front of the line once it's done.
I regularly use Uber from SDU to downtown Rio, to cover distances that I always walk when on vacation, but the heat and the business suit prevent me from doing so. Never had a complaint or a cancellation, unlike taxi drivers a few years back.