The most glaring downside for Mega Lucario ex is its Weakness to Psychic types, which time and time again have proven to be among the strongest in Standard
Going back the original example many paragraphs ago, let's look at the two cards you milled away: Milling the Gearhulk, if they are playing a Constructed deck, is likely an indicator they have even more Gearhulks
Final Fantasy IX Zidane and Vivi represent two of the primary party members of the game
Three-Mana Ramp Creatures If you want a 3-mana creature that puts a land into play, green has you covered with the trifecta of Wood Elves, Topiary Stomper, and Farhaven Elf, each of which excel in decks with lots of landfall synergies or 1-mana accelerants to curve this into
As far as Darkness-type cards go, Nidoking has the most HP of them all and a consistent way to deal damage each turn
Any pack without a specific category in its title should be considered unsearched