I once made a thread similar to this one before calli got nerfed.
These were the problems I had with it.
1. Number of rocket fired in the first salvo and time between when it was fired and when they landed was too fast.lead to quick wipes with little time to respond.
2. Sound of the rockets firing was too quiet. Often couldn't hear it firing from the fog of war.
3. High HP and armor makes it super hard to kill for a unit that never even sees the front lines.
When the enemy has indirect fire you have two options. Hold out and try to dodge the salvos which is hard to do due to 1 and 2, or try to kill it before taking too many casualties which is also hard due to 3.
It is expensive and it does leave the USFs with fewer AT units on the field. This weakness however is less prevalent in team games since they can help with the AT side of things and zooks can help prevent any stug or p4 rushes.
If I were to suggest changes it would be to fix problems 1 and 2 by making it louder and have less rockets in the first salvo to allow the enemy to relocate.
The sound being too quiet is an excellent example of why the best way to judge whether a unit is OP or not is by actually playing the game (vs just looking at stats). That sort of stuff will never be apparent if youre just stat-theorycrafting. I fully agree with it by the way, it IS very quiet, and miss it once and you're absolute toast.