I can't comment on the Nightwing thing, but Deathstroke seems to lose pretty badly to Superman. The last two WNF fights have shown the arguable best Deathstroke player getting manhandled by Superman.
This is the main thing that is wrong with this thread.
People are basing their opinions on locals in NYC and SoCal that have like 20-30 people average.....
The majors so far have been won by Doomsday, Batman, and second in both has been Aquaman. Yet because the weeklies are weekly everyone thinks that Superman and Deathstroke take priority because the good players from those regions are using them.
FYI, in the Canada major last week, CDJR got his Superman beat by the Doomsday and the Raven. It's not impossible, all it takes is matchup knowledge and competent players to pick up lesser used characters.