Ballistae are OP. Yes. Should they be removed? No.
Some reasoning: When I fall off a wall in siege, gravity kills me every fucking time. Gravity OP, remove please.
Also doors: they take really long to break, remove please, they are helping the defenders.
Honestly: if you are being focused by a ballista, you are predictable, slow, and therefor mentally weak. They really are not a problem. They help the defenders because, unlike real life, defenders do not have most of the actual boni that castle defenders had: Ladders do not require both hands and both feet like IRL, climbing a huge hill is not tiring like IRL, and pop is so low at the moment that without some very different spawn timer mechanics, defenders need all the boni they can get.