My point is, I want to see every civ to have interesting UA and 2 uniques that really offer something, they should bring some new playstyle or strategy and be different.
So as Rome, UA fulfills that, Legions change the normal path the other civs have, and bring really powerfull SW replacement, which changes the game.
But Ballista is not good enough, it's a slightly better Catapult. I want something more exciting and interesting. Some UB or some UU that opens new ways for Rome. If Rome would get Catapults and not Ballistas, I would hardly notice, it does not bring real advantage.