It is always difficult to evaluate anecdotal reports on statistical measures. I have always found USAir easier to redeem awards than any other airline I have tried to claim awards on. I cashed in all my miles on several different airlines one summer to take my extended family on a vacation together and found more availability on US than DL, NW or CO.
I have no idea how accurate it is, but WebFlyer keeps award success statistics and they claim that US is the best at allowing redemptions at 72%. UA is next best at 59%. NW is the worst at 37%. You can see the results for yourself at
WebFlyer