No offense, but everyone who thinks that there's nothing wrong with Undertaker is pretty much retarded. It's always the same, with every single card that got nerfed: "You can deal with it", "You can play around it", "just play another deck".
People said the same about Defias, 2 mana uth+buzzard, Tinkmaster and Pagle, etc
1)Even in the absolute best case scenario vs Undertaker, your opponent gains value or tempo from it- if you use Wrath or Frostbolt for example, you spent 2 mana to remove something that costs 1. The only cards i can think of that deal with it efficiently are Fiery War Axe and Backstab(when it's still a 1/2), and those are pretty much the strongest cards of their respective classes, CLASS CARDS.
2)If you compare Undertaker to the other 1 drops, it seems completely ridiculous. Like Zombie Chow, the only other neutral 1 drop that sees play outside of deathrattle decks. One deathrattle is enough to make Undertaker so much better. "Oh, how often does it happen that it's a 5/6" is a terrible defense because a 2/3 is already strong and at 3/4 it just starts getting ridiculous.
3) Now compare Undertaker to the other low cost cards that grow: Secretkeeper, Lightwarden, Murloc Tidecaller and Mana Wyrm. Murloc decks are just a more inconsistent version of Zoo anyways. All except Secretkeeper doesn't grow in health. You can't play nearly as many spells or secrets as you can play deathrattles. Also, all except the murloc can force to you play your cards ineffiently to make them grow, which brings me to 4)
4) So the only downside is that you have to include many deathrattles, which isn't really a downside at all, because Webspinner, Haunted Creeper, Mad Scientist, Harvest Golem, Piloted Shredder, Belcher and Highmane are already the best cards in the game for their respective mana cost. With Undertaker you just have to play the cards you would've played anyways.
tldr: everyone who thinks that Undertaker is fine and won't get nerfed is out of their mind. Let's just hope that blizzard doesn't need a few months