With one ballista you can usually take out one tower and break one wall segment if you don't have bad luck. Properly positioned that helps a lot. It is a bit unrealistic to shoot down stone walls but it is necessary for the siege mechanics of TW games.

I battle artillery poses a problem. At least for hellenic factions it is a kind of "I win" button because it forces the AI to attack even if you are the attacker. I think it's not possible to design because of hard coding but I would make ballistas and onagers immobile in non-siege battles (they are huge and heavy and don't have an engine), make Cheiroballista/Scorpion and Polybolos mobile (the latter is not, presumably for balance, nevertheless a bit strange and not necessary) but very slow. They should not force the AI to attack. While the latter is not totally irrational and unhistorical (for example at Mantineia 207 BC the Achaeans attacked before Machanidas' Spartans could use their catapults in the intended way), it usually puts the AI into the inferior position.