If the target creature dies but that card leaves the graveyard before the persist ability resolves, it won't be returned to the battlefield
This can give them a huge advantage in the early game, allowing them to establish a strong board presence before their opponents have a chance to react
Also, once again, in 1v1 gameplay, you yourself will be one of the 2 players, meaning your equipment, controlled and owned by you, will fall off the creature because it can't be targeted or equipped by your stuff as soon as it has protection from you, making it quite the useless sword
Bad Mechanic Hosers (Nine-Ringed Bo, Tombfire, Root Cage, Trapfinder's Trick, etc) There are a number of mechanics Wizards of the Coast were clearly worried about when they printed them
Being only a couple weeks out from Ugin's Labyrinth While this list is in no particular order, I figured I would start with Ugins Labyrinth, which is arguably the most powerful card in the set