IMO, jags and things with brass threads have a good feature and several bad features.
The good feature is they cannot rust or corrode and get "rusted in" in a cleaning rod like steel threads have been known to do.
The bad thing is the strength of the brass or bronze is almost always less than even the lowest strength steel. (Some alloys are the equal of low carbon steel but they are rather special and unlikely to be used on a cleaning jag).
Another bad thing about the copper based alloys is they will "work harden" and become brittle if they are subjected to repeated stress.
(Every time they are used, they are subjected to stresses).
New brass threads are fairly ductile and unlikely to break.
Well used brass threads can become brittle and very likely to break.
I think this explains why some folks have not seen a problem while others have.
I know I've had several that broke. That's the reason I take the trouble to replace the threaded portion with a stainless steel set screw.