Undertaker was not mildly OP. The card was disgustingly good.
Blueblimp pretty well explained why Mad Scientist is a messed up card to have in the game. The alternative to him was to just make better secrets over time, but he was a quick and dirty and ugly card. And he makes games swingy as hell, it sucks to draw him after you've drawn all your secrets even when running the correct # of secrets, it sucks to fight someone who played him on curve, just kinda suck sucks.
But yeah Haunted Creeper and Sludge Belcher seem like pretty good cards for the game I won't disagree with that. I wouldn't disagree that Naxx isn't better than classic, I just excluded Classic earlier because it seems too hard to compare with expansions that don't have a before and after story for people who weren't in alpha.
Any OP cards in the upcoming set (which seems more doubtful than Naxx), is going to at least be OP in a healthier way that can be countered with the right strategies. The answers aren't going to be "use your kill spell on a 1 drop lol" or "hope your opponent draws the sh#$%ty secrets instead of the power card", they're going to be "build a deck with a strong board presence that can last through Reno's heal" or "keep a Fireball for the monkey and apply pressure", or even, at the worst, "owl the rogue's loot hoarder", which isn't nearly as awful as Naxx's nut cards.