It's not AMAZING in CW but since all you'd take out is a Shieldmaiden it's not like you're losing a Sylvanas or Emperor or whatever. And while it doesn't enable you to do a 7 mana shield maiden + 5 health removal slam combo it does enable you to do a 3 mana 4 damage slam removal every turn thereafter. I don't get how you can say it's not that good.
Shieldmaiden has a better body and can fight for the board. The hero power does not provide any board value or allows you to generate card advantage the longer the game goes on. It's already established that this will most likely die to some scrub minion or 2 mana removal, so you really need to get some nice value down the line.
Stacking up armor is not value, you will still lose if you can't take the board and your hero power will not help you in doing so. As I said, the best thing it will do is buy you a turn or two to draw something else if you are losing and absolutely NOTHING when you are winning. Something like 2 dmg ping or 4 heal can generate card advantage, similarly buys you time, but by actually killing stuff or preserving your minions instead of facetanking it. And when you are ahead it secures your victory instead of doing nothing.
The shield slam activation is cute, but you already have 4 big activators plus your hero power so you really do not need another for it. You almost always get your desired Shield Slam right now, no need to commit to enable it any further. Shieldmaiden does the job just fine and doesn't die for free, allowing you to actually take the board and win.
You have to be a fool to play this over Shieldmaiden in CW.
10000 armor won't save you if you don't have cards/board. The only time this actually matters is in matchups versus burn decks like Freeze Mage and Face Hunter. Incidentally, those are actually your very best matchups already and close to autowin, so you really don't need to weaken your overall performance to do even better in these MUs.
Very underwhelming in CW imo.
