Good question. 3000 PSI in no joke! Most of the time on "healthy" cast iron pipes it wouldn't be a problem. However, if your cast iron pipe is weathered, pitched wrong so it has standing water (thin wall from erosion), you could punch a hole in the walls, leading to leaks, root intrusion and other problems.
I always send my camera down the line and do an inspection first.
My neighbor next door to one of my apartment buildings every year had root problems and back-ups, 150 feet away from a cleanout. So free of charge I cleaned out his main sewer line then used a camera to determine the condition of the line. I could see the pipe was not straight, roots had caused the pipes to move leading to leaks and roots. I used a foaming root killer so his root problem would take 2 years to reoccur.
But his son didn't want to rent a cable machine and rented a hi-power jetter. He opened what was smaller holes into large holes and caused root problems sooner -- now every 3 to 4 months.
I then said I could run an epoxy patch from the cleanout and fix the leaks without digging up the pipe. His son instead dug up the driveway and replaced the sewer pipe.
So instead of a $1000 patch, they spent $2000 on digging and replacing the line and paying a asphalt contractor to repair the work area, another $2,800.
--47.156.xx.xx