Back in Texas, a crew I knew used steel core rods for pulling cable through conduit under a parking lot. They swore by them for long pulls, but one time the rod snapped near the end. Had to fish it out with a magnet on a string, took hours. Made me think about trade-offs, stiffness vs flexibility. Still, for most jobs, they're solid.
Snapping a rod near the end sounds like a nightmare, fishing that out. Derek_bailey84 has a point about them not kinking, which is huge on a long push. I read a forum post once where a guy said the steel core can fatigue if it's bent at the same spot too many times, maybe that's what happened. Makes you wonder if a slightly more flexible rod would've just bent instead of breaking clean.