I can tell you that the turnbuckle on a J/24 will part explosively if you're tightening up the shroud when you think you're loosening it, well before the shroud has any problem. Don't ask me why I know this. :-)
Yes, stainless
turnbuckles can and will fail from crevice corrosion after time and if they're 20+ years old, it might be time to either have them xrayed, dye checked, or simply replaced. On a 19'er there's a limited amount of damage you can get into by losing the
rig, but still, it could really ruin a nice day on the water.