I like the card.
I don't necessarily agree that pinning is a problem. You don't have to discard anything if you just buy treasure. "You get pinned, unless X" is allowed on official cards, e.g. torturer, but I notice that every fan card anyone suggests that is almost as brutal as torturer gets criticized while Torturer itself is beloved.
Torturer is not beloved. Its attack might be fine on a card that didn't draw so many cards, but it's too harsh as published.
"You don't have to discard anything if you just buy Treasure" is the real problem with Poisonous Arrows. Attacks that target Action cards are a Bad Idea because you can thwart them by having a boring deck. Yawn.
I'm behind this 90%. Not sure whether Torturer is "too harsh".
Either way, Torturer at the very least has two attack effects, and lets you choose which you do. What's more, the harsher effect enables you to do the less harsh effect without any problem - so chaining Torturers means that only 2/3 of them hit.
If you make a card that targets actions, it's very likely a bad idea to keep it without bounds. You could still have it be a choice, or maybe even the bane of the attack (edit: "bane" is commonly used to describe something you can do to be spared from an attack, named after the "bane" of Young Witch). Not sure whether "each other player with 5 or more cards in hand" fixes the issue - if i have only one (very good) action in hand, and another opponent three cheap ones, the attack hurts me so much worse. So, the attack would affect decks build around fewer stronger cards more and become very frustrating to players going for them.
Here's a Looter attack idea i had a while back. The original gained to hand, but whatever:
"Each other player may discard an action card. If he doesn't, he gains a Ruins"
