I can't say I've tried all card games but from the ones I know and did try this is the one I ended up liking. I've seen plenty of steam reviews stating the same thing as you though. I'm a fairly new player, but from what I've gathered older sets aren't obtainable anymore with gold but only through seals which makes it really hard to complete older decks without investing real money?

If you're talking about "Might & Magic Duel of Champions - Starter Pack" - that's not a Steam key. Here are the instructions for using that key - not quite the same as clicking the "Activate a Product on Steam..." menu item... :)

"Might & Magic Duel of Champions - Starter Pack - Account Information

How to redeem the Humble Bundle code of Might & Magic: Duel of Champions

Go to https://www.duelofchampions.com  
Download and install the game  
Start the game  
Login with your Uplay account – or – create a new Uplay account  
Go to the in-game “Shop” menu (top right button)  
Click on “Redeem Code” (bottom right button)  
Enter the code inside the entry field and click OK  
Go to the “Cards and Decks” menu  
Click “Open Packs” to find the packs you just unlocked"

:)

If you go to the bundle page and look at the icon underneath this item, it's a key. If it were were Steam-redeemable, it would have a Steam icon (as several other items on that page have).

The game is playable on steam or outside. The starter pack from the bundle it's inside the game, not the steam one. So you should remove this giveaway.
You can check my profile, I don't have it. I activated 3.

Also the booster packs included aren't all (most but not all) the same ;-)