Inferno is busted in the UR counter burn style decks IMO. It comes down and the game is over. Free lightning bolts are disgusting. Prime Time gets manlands and other busted nonbasics, so it's too easy to abuse. Sun Titan can sometimes run out of things to reanimate, but the effect has so many good targets typically and you usually don't need more than a couple to pretty much bury your opponent in tempo. He has vigilance too which is a very underrated ability IMO. Being able to apply pressure but still have a giant 6/6 wall to block is crazy good.
Again, Frost is the only one that isn't just silly over time. He permanently locks down your opponent's best resource, and it's virtually impossible for some deck's to kill him. But he never nets your more than one virtual CA. All the others provide exponential value over time, which in a slower meta makes them considerably more broken.