I have a hard time knowing if these comments have their roots in a genuine concern for safety or Randy Wakeman style fear mongering.
I am a woodworker and freely admit my knowledge of metalurgy is lacking. I know the pressures on the breech plug are very high but to me the ventliners appear safe in design, (I don't see how the head of a ventliner could ever come through the hole and come back and bite you), much safer than a nipple threaded in from the other side.