Thinking a bit about the differences between the physical and digital versions over the past few days, I think this is what I fear the most so far.
If you are out of the loop on that one, Sneak Attack in the digital version works pretty different. I think caleb mentioned it during the annoucement stream. What it boils down to is that currently it kinda works like this:
QuoteCost: <unknown>
Action: Put into play any ally from the game until the end of the round. If it is still in play at the end of the round, discard it from play.
Now this fits into what Caleb said is exciting about the digital card game: more chance to introduce randomness. Now I am not against the randomness in the card. What I really dislike is how it can pull any ally, regardless if you own the card or not or if you did put it in your deck or not. I understand that this can be exciting, but I also see why the card is included. Hearthstone uses this mechanic from the start and IMHO the prime reason is to show you cards you do not own yet and show you how powerful they are in an attempt to make them more attractive to you.
Now I think this is a deeper problem than most realize. I see 3 distinct problems:
Player Trust
I don't expect FFG to release chances and mechnics of the cards to players. With that, you can never be truly sure on if Sneak Attack will not be biased to show you cards you haven't aquired yet, cards that recently released or just cards that fit the current quest/difficulty. I don't think there is anything FFG can do to remedy this. With adverserial games like Hearthstone this is less of a problem as Blizzard has a large incentive to keep the game fair. In a coop game, this incentive is gone, as the encounter deck doesn't care if some players get an easier or harder go at it.
Limiting Deck Construction
By pulling from all cards, your choices in which cards to include in your deck do not matter any more. This essentially makes Sneak Attack an anti-combo. You can not rely on it to give you something specific. Which is why it does not matter what else you put into your deck. This makes it fit in every deck archetype. Also this creates the additional problem of the card being either too good value so you probably have to include it in most decks or not enough value making it irrelevant in most decks. Both is bad.
Cutting Design Space
A card that can fetch every ally released can break a lot of quest designs. In theory quests in the digital game could restrict the cost of allies to a certain value or value range. Sneak attack would override that by default. Also mechanics that punish unique allies or swarmy decks might be disrupted by Sneak Attack. To be honest, I am not that good of a game designer to see all the implications right now. I just know that in Hearthstone, similar cards were the ones that meant trouble for the game. Thanatos, Mage Portal and Shredder come to mind.
Is there a solution to this? Through secondary channels I was able to bring this concern directly to Caleb (albeit in a limited fashion), and the response was that they are just trying it out. Some tweaks include pulling allies just from your collection or your deck. I would very much prefer to limit it to the player deck, but that also means the flair of the card is mostly gone. So I am not banking on them going that route. Pulling from your collection is promising, but might enable optimization strategies where you try only to get very few allies into your collection but these are very powerful, essentially modifying the value of Sneak Attack through your aquisition strategy. Although this would also mean a tradeoff for the player in reduced deck construction space, it might be too strong to pass up.
So what are your thoughts? Is this card the problem I see in it? or is it largely okay and I should just not worry?