It is pretty damn strong, but keep in mind it's not 100% accurate, it's just much more accurate than normal. There definitely are plenty of times when the shell misses and doesn't kill the squad.
Counter barrage is actually very strong too. Not for countering precision strike, but just overall it's very strong in mortar duels.
In my 150+ soviet games, I have never had precison strike miss.
It's not overpowered, a unit shouldn't be idle for that long anyways, if you know they have a 120mm mortar, and that they are vet 1, then you shouldn't leave your units idle, and move and re deploy.
Why does everyone keep bringing up the 120mm mortar? The regular mortar from T2 has the precision strike ability, and fires the 120mm shell when using it. This has nothing to do with the call-in unit.
And a unit shouldn't idle for that long?? It takes 5 seconds for the shell to hit. What kind of setup unit stays setup for less than 5 seconds?