For sure they have drawbacks. But Cersei knows they are coming to kings landing. I would have lined the walls and every tower with ballista. If they are already in place and mounted correctly you can have fairly good range of movement to aim with. Also highly effective against any siege towers or ships in the harbor. Taking out the dragons and ships should be top priorities.
Are you thinking of scorpions? Basically they launched bunches of arrows at once. Highly effective against massed infantry and cavalry. Much lighter and easier to move.
Yeah they were great in Rome Total War...