Some simple calculations for anyone who believes Dune Scorpion is hard to counter.
Player 1: 6 dune scorpions, 6 momentums, 18 other cards
Player 2: 15 pillars, 15 hitters (average rush deck)
Player 1 plays a momentumed dune scorpion, 2 cards, 4/5 quanta cost for 1 growing damage each turn + Neurotoxin.
Player 2 plays on average one pillar per hitter, so that's 2 cards per hitter. 2 poison damage on yourself, 5-15 on opponent depending on your hitters (frogs - ruby dragons). Ok, if you go for dragons you could play 2 towers per dragon, it's still 3 poison damage for you 10-15 damage for opponent per turn. If you go for cheap creatures on the other hand you could play less than 1 pillar per creature. If you play nova/immolation, it's just a replacement for pillars, card count per creature staying roughly the same.
Conclusion: Neurotoxin by itself 'deals back' less than 1/3 of the damage you do each turn. On average, against a rush deck. Killing a rush deck with Neurotoxin itself is nigh impossible.
But... of course the Dune Scorpion player has those 18 other cards in his deck. And momentumed dune scorpions deal 1 damage + 1 more poison each turn. That's roughly equivalent to a physalia, but with much higher initial cost (2 cards, 4 quanta against 1 card 1 quanta), but without the recurring ability cost. If we assume that an average physalia uses it's ability 3 times during a game then it seems that (not counting Neurotoxin) the 6 Dune Scorpion + 6 Unstoppable is equal in power and cost to 6 Physalias.
Conclusion 2: We are 'paying' 6 cards for Neurotoxin.
Conclusion 3: We are paying ~1/5 of our deck to 'return back' around 1/5 to 1/3 of damage per turn our opponent deals. Seems a bit powerful, but generally balanced, especially because if opponent goes first, he will play most of his hand (mostly pillars) before we set up the combo. And there is no guarantee we can set up the Dune Scorpion combo during the first few turns.
Conclusion 4: Against non-rush decks, it greatly depends on the deck. Quantum denial can stop the combo by draining
or
before a single scorpion is played. Healing decks can outheal poison. Most control decks can kill the scorpions thus suffering only from Neurotoxin.
What about other decks? Those that cannot outheal poison damage, nor prevent it with denial? Well, those are vulnerable to ANY poison based deck, so they should pack Purify if they have access to
, or if they don't they have to deal with it. Some decks are vulnerable to poison.
Conclusion 5: IMHO Dune Scorpions are an average card, neither too strong nor too weak. Usually they are more of a nuisance than a game breaker (the previous version of Dune Scorpions was even better at this). Decks with a lot of scorpions/momentums are vulnerable just because they use so many cards to set up this combo fast. Decks with less (eg. 1 scorpion, 1 momentum) need to dig through the whole deck to set up the combo giving you ample time to finish your setup and kill their owner. If I were a stupid False God, I'd be afraid of 'em. But I'm not