I’ve also never heard of a shrapnel-based injury. Only door falls or torsion bars breaking arms.

I’ve actually replaced these in two houses, and yes, they’re dangerous, but honestly a little common sense and a straightforward (printed, with a buddy double-checking) list of steps you take with each quarter turn.

It’s super important to have the right torsion bars. Don’t try to invent your own.

I'm far from an expert but many (most?) kinds of spring are hardened steel. It's not hard to imagine how they might shatter under the right stress.

Anecdotally, yes, it happened to me. An older extension spring setup where each end of the spring had a J-shape hook. The hook itself sheared off of the body of the spring and shot a ~2 inch steel boomerang across the garage. Fortunately my only injury was psychological and temporary.

The replacement spring was constructed differently, with the "hook" portion replaced by two complete loops of metal. The safety cable runs through that and would hold it back during a similar failure.