You know, I've always been confused as to why Miracle will sometimes only run 1xConceal. I don't play constructed, but I always got the impression that you wanted to do Auctioneer + Conceal to prepare for next turn. Maybe I'm overestimating the amount of mana you need to go off, but having to spend 5 on Auctioneer feels like a lot, and leaving an unstealthed Auctioneer sounds like suicide.
The problem with Conceal is that it's kind of a "win more" card. It's totally useless in your hand most of the time if you don't have an Auctioneer. Some people run 2x Cold Blood and 2x Conceal, and those decks seem strong when you play against them and see them go off, but a lot of the time, they're going to find themselves with literally no playable cards in their hand.
Even without Conceal, it's not that hard to draw 3 cards off of an Auctioneer. Prep+any spell draws 2 cards for zero mana, and then if you have a couple more spare mana you can play another spell. Then either you've drawn enough to kill or you have Conceal for your second Auctioneer. The thing that makes modern Miracle decks so much better than the old ones is that they don't rely nearly as heavily on Auctioneer+Conceal because they have more minions.
Are there any legitimately competitive decks that can use Jaraxxus? There are zero with KK in it.
Jaraxxus used to be played in both Handlock and Zoo, but is now played in neither.
The problem in Handlock is that on turn 9, you need to be clearing the board and/or playing a taunt so you don't die to Force+Roar or Leeroy+Shadowsteps or some Rockbiter or Bloodlust thing or whatever. Being at 15hp isn't any good. So the only way you can play him is if you can clear their board with Soulfire or you have some taunt still up. This doesn't happen often enough to be worth it.
The problem with Zoo is the Doomguards will always discard him. I used to play a version of Zoo that just maintained board control until turn 9 to play Jaraxxus and then win with the Infernals. This doesn't run into the problem of needing board impact on turn 9, since you should have board control at that point. But this plan is inferior to the "win with Doomguards" one.
The real problem with King Krush is that Ragnaros exists.
The problem with King Krush is that:
1. It's a control card that's only usable by a hero whose hero power is not for control strategies.
2. It's the greatest thing ever for your opponent to Faceless.