What?
Celestar Ballistas do not have better firepower. They have ****** range of 18", or only 1 shot on range 36".
Celestar Ballista is 110 pts and does on average 1,30 damage with range 36". That is 0,12 damage/point.
Celestar Ballista does on average 2,60 damage with range 18". That is 0,24 damage/point.
Reaper Bolt Thrower is 120 pts and does on average 3,33 damage with range 36". That is 0,28 damage/point.
And in ranges between 18"-36" it is better to take 4 Ballistas than 3 Ballistas and an Ordinator.
And if you do not care about the max range, Waywatchers have treat range of 26" and do 0,31 damage/point. Sisters of the Watch have treat range 18" with the same 0,31 damage/point, if they move 6" they have treat range of 24" but it halves their damage. The advantage is that the Sister get +1 to wound against Chaos units and their damage potential goes up to 0,38 damage/point.
Well, I don't care about the Ballistas and my Stormcasts hire the elves to do the shooting. You had to take 4 Ballistas and an Ordinator to make Ordinator buff to be worth it and to be better than the elves (Bolt Thrower, Waywatchers or Sisters of the Watch) and I am not up to that kind of investement.
Ballistas are much more resilient though, but I never had problems with resience of Bolt Throwers, I try to keep my opponents busy taking care of more threatening targets like 10 charging Evocators.