Ok, so look.. One of the issues with poison.. It just happened to be something I was reminded about when watching a Gwent game.
It's how damage is priced, and how poison is by far the most powerful damage in the game. Perhaps this is even the root of the issue.
So, in the game, the person boosted a mawler to 6 power and 2 armour using radovid loyal guard (isn't that the name??). So, the opponent used Fisstech Trafficker and Mutated Hounds to do 8 overall damage, 6 body+2 armour for a total of only 8 provisions, while putting 6 body on the table and 1 armour.
I don't know any other two bronze cards in the game who can deal 8 damage to a unit while putting 6body and 1 armour on the table. If we take a closer look, even 4 damage is something mostly reserved to expensive gold cards. Even 10p gold cards! Yikes. The sheer amount of damage that poison can do is absurd.
Sure, it is over 2 turns, but unless your deck is a purify deck, there is nothing you can do to stop it. To do the amount of damage that poison can do very cheaply, would require a very heavy control deck, with the best damage units in the whole game, combined with bronze damage cards. A real hardcore control deck, might be able to output as much damage as poison during 3 rounds.
But the cost would be significant, in provisions, and to your deck building. It would cost ALOT of provisions, and it would reduce your deck to only damage dealing units, and still it might not even be able to output as much damage as poison.
So, the problem might essentially be poison being the cheapest damage in the whole game. Even just dealing damage with a unit on a unit with 4 body, usually requires expensive gold cards.
Unless you only play units with 3 body or less and spam the board with those, poison will be able to output more damage than any other deck. As much damage as a "special card only" deck, but WHILE putting bodies on the board at the same time.