Yanmega is a really good card, I'll admit that. But at 110 HP it's insanely squishy. Reshiram and Zekrom can one shot it, and Magnezone can also one shot it given you have enough energies to LZ.
Reshi/Typhlosion decks have a strong advantage over Yanmega IMO, since you can essentially do 120 every turn for as long as you can keep your incoming damage low and have a Revive at the ready.
Which brings me to the subject of Yanmega's strongest and weakest point: A limit of 8 hand equalizing cards in the deck. In a best case scenario, you could do 70 for nothing every single turn for 8 turns once you have Yanmega up.
However, unless you have god mode luck or you stack the deck, getting a hand equalizer every turn is quite a feat to achieve. And that's where Yanmega gets caught. Unless you have a way to get quick energy to it, you basically have a 110 HP bug that can't attack.
And with Pokemon Reversal in the format, even retreating can't really guarantee he'll be up next turn.
So while I do think Yanmega is definitely among the top cards in the format, it's not as invincible as people hyped it at Nats, and it's going to have a very short run after Worlds has ended.