Just because a deck doesn't win Nationals doesn't mean its a bad deck. Blaziken, for instance, got 2nd place in Nationals. If you think that's bad....wow. Most players won't ever do that well. Anyway, there is a matter of how what is popular or not. Imagine if:
-200 players played Gechamp
-75 players played Blaziken
-60 players played Absol
-20 players played Rayquaza
-100 players played any other deck.
Statistically speaking, which deck is more likely to get to the top? Gechamp, mainly because that's the most popular deck. Sheer numbers can sometimes effect of how well a deck fares in a tournament.
Am I saying Gechamp is a bad deck? No. When a deck wins Nationals, it is usually a sign of it being good. The main point I'm trying to get across is that we can't always choose one area's results for determining how good a deck is. We'll have to wait until BRs to really know how DP-SV pans out.