Ive read several articles to that effect also Doug, hard to pin it down to one single cause. As your probably aware the HiPo 289 had a much larger Damper plus an extra counterweight that sandwiched between crank gear & front journal. So Ford more or less must have acknowledged a problem. I 'think' there was an issue with balance at continous high RPM, articles from GW & other builders support that & GW went to some considerable efforts in Main Cap girdles that tied into the pan rail.
The nodular iron crank simply wont stand up to the abuse of a steel item and the 289 block regardless of whether its a HiPo or std item does not have a lot of metal in the main webs to prevent flexing.
On top of that you hear of teams/drivers that had excellent durability while others were forever in trouble....part of that could come down to the quality of the balance work, but some drivers 'knew' how to 'breath' the motor on long straightaways etc to avoid harmonic vibration buildup. When you read some of the comments from guys involved in the Shelby race teams about the 'quality' of some stuff provided by Ford it really makes you wonder if they were even interested in good race results. Little things like still running the factory pressed piston pin setup & big things like supplying factory ready-do not touch- tunnelport 302's that had a misfire on first start and found to be missing pushrods etc...makes you wonder...but then perhaps I shouldnt, I worked in a Ford dealership for three years, In that time I filled about six large foolscape folders with service bulletins of problems & cures,.... GM are/were no better, subscribed to a newsletter called chevy hotline for a few years- lots of 'production' faults to avoid there also, remember the first 'Tin' block my old Boss bought for his Camaro, lifter gallery oil drilling did not meet in middle of block & at that point was so low that it 'missed' the waisted area for oil on the lifter body, if not found at assy that would have resulted in a 'dry' pushrod/rocker/valvestem, but a quick burst with the diegrinder & it was all good.