Poisonous Mushrooms is great against enemy allies with 1 or 2 HP, though. If you have played your mushrooms before the enemy plays their low-HP allies, you can usually kill them before they can attack (or one turn after they attack). Another great thing about Poisonous Mushrooms is that it can bypass the ability armor, since it's not possible to poison allies if there's no damage dealt to them, even if your Venomous Cobra blocked you opponent or was defended against. It's a guaranteed poison. It can neutralize an ally's ability to regenerate, although it's not the best tactic to hit a regenerating mob without killing it off. Still, poisoning it can at least stop it from returning to its original state.
Strong Against: Aggro and armored decks.
Weak Against: Object removal and basically any ally with 4 HP or more.