The issue with poison is not the status itself, but rather how most poison are in the 4 provision range, making them too easily accessible, it's particularly the case for Syndicate which has access to 3 different poison cards at 4 provision, so 6 poison cards in total.
In the case of Scoia'tael poison being at 5 provision means you have to chose if you want to build around it, but for Syndicate, poison being so cheap means it doesn't cost you much to just slam all 6 cards in your deck and call it a day, it's true to a lesser extent for Nilfgaard and Fangs.
Because you need to poison twice to destroy a card, you always want to have two of those poison bronze, nerfing them by 1 provision each (and eventually readjusting their power) would mean 2 less provision for the entire deck, which is huge, and therefore you'd have to carefully consider if you want to play poison for a more control approach, rather than playing poison because it's just good value.