Not having ever worked on anything like this, the death knell for these projects must be the inability to ever reduce functionality into a reasonable core set. Basically the 80/20 rule but where there are practically an infinite number of edge cases that stretch the project on indefinitely? You could serve 80% of the payroll burden with software that cost $10m. 90% coverage costs $100m. 98% costs $1,000m. And 100% co…
Yes, people do make huge blunders in powering forward with overpowered, unnecessary solutions to problems they largely do not have; but blunders this big usually don't take this long to suss out.