In 39 years of shooting, buying, being issued, competition, and examining literally thousands of different K-frame magnums, I've seen one cracked forcing cone in person. It was two years ago. My gunsmith had a snubby 19-5 come in with one.
I own dozens of K-frame magnums. I shoot 158 grain 357's through them, and I clean them afterwards. I've yet to "shoot one loose", or crack a forcing cone.
I am puzzled that you haven't read about this alleged "issue" with the K-frame magnum. Every newb who buys a 19 or 66 usually frets about it and posts asking if "brand X 357's are ok to shoot through their model 19?"
The L-frame fans always mention this alleged "K-frame magnum flaw" as the reason S&W came out with the L-frame. They never seem to be able to explain why S&W continued to produce the K-frame magnum right alongside the L-frame for twenty plus years, but no matter.
On the internet I've seen photos of cracked forcing cones. All the guns pictured were model 19-5's. They were also filthy dirty. The 19-5's were also the first 19's to get crush fit barrels. IMO S&W took awhile to get that cost cutting measure right.
That, combined with poor or nonexistant cleaning, and the use of hot light weight (125 grain ) 357's is what causes cracked forcing cones, IMO. YMMV.
I never have heard of it happening with the stainless K-frame magnums. Maybe carbon steel isn't as tough? I seem to recall someone here posting about seeing an LEO model 13 with a cracked cone. TJ