This ability is so powerful because it's meaningful at any stage of the game, whether you need more mana to ramp out the rest of your hand or more cards to finish off your opponent
In previous years, Id dismiss this kind of card, as the time and mana it took to cast it was less meaningful in games that regularly went over 10 turns, and with decks that drew far fewer cards, on average
Mechanically, there are few things it can do that other colors also can't, making it feel like dilute and less focused than blue, black, red or green
With multiple Greninja BREAKs on the board and some energy in your hand (Fisherman, anyone?), you can do some serious damage to your opponents Pokemon to take some easy prizes, and thats before you even attack