For starters, the top end is absurdly powerful. Simply by performing Legion, you have to means to do any one of at least two of three things: stand upon hitting, retire two upon missing, or sitting on the Grade 3 you just grabbed as a means to feed Stride plays on the coming turn. And all of these feed into one of the strongest Overlord support cards - Dragonic Burnout, which is as close to free as retire gets. The fear factor is strong, and the control factor is utterly devastating.
But of course, things don't stop there. The rest of the deck is also a well oiled machine. You have all sorts of options at Grade 2 besides Burnout, including Berserk, Twilight Arrow, Neoflame, and ye olde Burning Horn. Grade 1 features the absurdly powerful unflipping perfect guard, Calamity Tower, Gojo, and Lava Flow - and those are just the usual suspects. Nothing's stopping you from using Grom or Violence Horn, for example. Red Pulse and Fargo are both strong forerunners, and of course Gatling Claw is an eternal thorn in the side for every deck depending on their own.
So what you get after all is said and done is the best of both self-advantage and opponent-dismantling, and you can shift gears depending on what the opponent is doing in order to assure a good outcome for yourself even if one of these factors is being shut down somehow. Not many other decks do that, nor can many other decks put up with that. At least, that's what the results would suggest.
Naturally, I don't believe it's quite as dominant or even as problematic as the masses might. The state of the G format insofar is roughly that of where the game was BT05-BT09; there's a clear favorite, but plenty of other things see regular tops and can play against the favorite on relatively even ground, namely Royal Paladin, Shadow Paladin, and Oracle Think Tank variants. There's no doubt in my mind that Neo Nectar will perform perfectly well too, although the remainder of G-BT02 is still up in the air as far as viability goes. Aqua Force, Narukami, and Great Nature have historically lagged behind for a lot of reasons, and while Nova Grapplers aren't quite so prominent at the moment, they're still somewhat of a tall order to match.