Everything counts, starting with determining how far from the top of the steerer that the threading actually extends!

Plenty of bikes were made with a LOT of threaded length on the steerer, certainly enough to where the "safe limit" line on the stem quill would not assure that the the expander was in an unthreaded section of the steerer.

I think that the quality of the stem plays huge as well, as some stems are vulnerable to having their slot-bisected lower end literally crack and collapse (with complete loss of bolt tension and steering control).

Rider weight, stem/bar leverage/length, riding conditions, and length of service, again, can all play huge into the reliability equation!