I once "won" a trip to Hawaii.
IIRC-- The gist of it was that you left on a redeye, had two nights in a mystery hotel, and left very early on the third morning. Any alterations to that required that monies be paid. At the end of the day, I threw away the voucher and vowed never to be lured in again by such things.
I still marvel that miles and points travel is as wonderful as it is, and is not a scam or a loophole for anyone willing to put in a little effort and keep their credit card bills paid in full at all times. It is simply amazing and I'm so grateful.