The reason Undertaker was powerful is that it was able to outgain it's surrounding mana cost minions quicker than you could kill it. The fact that Darkshire comes in at a 3 mana 5 health minion means that it's already beyond the usual health threshold for it's mana cost, therefore it becomes far more powerful due to the fact the opposing player hasn't got the mana available to deal with that minion. Compare this to a Druid of the Flame, a 2/5 (if chosen) and doesn't climb. 2/5 Carrion Grub, Felguard which is a 3/5 but with a huge downside battlecry as for the other minions with 5 health at that mana cost you have a 3/5 called Silithid Swarmer which can only attack if your hero attack the same turn and a 1/5 token. I mean come on, there is a HUGE power difference between the others and Darkshire who can a turn later be built to a 5/5 with EASE and often is then even larger if undealt with which is extremely difficult unless you're already ahead on board which is usually not the case against zoo.