I don't know if I've been "stiffed." I don't keep track of who "owes" me a print, but I do keep a list of who I owe a print to. Sometimes it takes a while for me to get prints out, but they do get out eventually.
I did back out of an exchange once on another forum, because I just didn't have time to make the deadline, but by the same token, I didn't receive any of the other prints in the exchange, since the organizer collected all the prints into portfolios and then redistributed them to those who had sent a print.
I suppose if we wanted to make sure the exchange was always equal, we could send all the prints to the organizer, and when they were all in place, the organizer would send them all out in this way. Maybe those who are really worried about this aspect of the print exchanges should establish an exchange in this form and call it the "Guaranteed Exchange" or some such, where no one will receive any prints unless they send one in. It would be a lot less unpleasant than acting like a collection agency.