It suffers from being a top end card needing landfall triggers like Emeria Angel
Unless they play the 100HP Mewtwo from RUMBLE or attach an Expert Belt, Garchomp C Lv.X ( C) can answer a Mewtwo at least once, but the opponent can repeatedly try to get Mewtwo on the board which can be difficult if they also have other attackers or are disrupting you
Allow me to break down the benefits step by step
The most recent set, Obsidian Flames, for example, leaned so heavily on Charizard, that, despite having some other great cards, it struggled to take off, and Master Sets have been selling for under $500
Sure, your opponent gains life, but isn't that a small price to pay considering you're only putting in a one-mana investment