While I was doing some research, I came across a detail, that as of Emperor Trajan, most Roman Legions were equipped with 55 Chieroballistae, basically to my understanding mini-ballistas that fired very large sized bolts far and fast. Aside from any other siege equipment. That struck me as quite interesting, because that is presumably a lot of firepower, of the kind that (as I understand) most other powers weren't able to field.
Yet, as a casual history buff, I've never come across any battle where the presence of these field/siege weaponry was at all credited with... anything, really. You would think the fact that the Romans could at any point deploy dozens on dozens of siege equipment to dish out murder bolts from afar would have been a more deciding factor in battles than it seems they get credit for.
Are there any examples of battles/conflicts where specifically Roman crewed/siege weapons were of particular note or use? Were they not all that unique? Or is the reason I haven't heard much about them just the usual Roman bravado of "who cares about throwing stuff around, the battle is all about heavy infantry clashing and the will of generals".
I wrote a reasonably lengthy post on this topic a long time ago, here
The figure you mention probably comes from Vegetius, but as the linked post discusses it is questionable how far we can trust him. He's describing a theoretically idealised Roman army of centuries before his time, and we do not know if such a reality ever truly existed. It seems quite likely the number of available artillery pieces differed from time to time and legion to legion.
As for effectiveness, you are probably thinking in too modern terms when you're talking about "firepower." Even 18th century artillery, though vastly more numerous and powerful than Roman torsion artillery, rarely decided battles on its own. Roman artillery would not have been able to inflict all that much damage even if dozens were present in a battle involving tens of thousands.
It is true that our sources mention them very little, but it is probable that even in battles where they were present they would only have fired a fairly small number of stones or arrows, and perhaps killed or injured a few enemies. But that does not mean they were ineffective. Being shot with such powerful weapons from ranges beyond anything a Germanic tribesman or Parthian horse-archer could manage (see the descriptions on the effects of ballista shots in the link above) would certainly have been quite frightening and demoralising for the enemies being shot at. Even if these weapons never decided a battle, they may still have had an effect in intimidating the foe.