My friend and I built a number of flat tappet engines 20 years ago and I'm sure most didn't start right away. I doubt we ran them at 2000 rpm for 20 minutes either. We would have used ordinary cheap oil for break in purposes. There were no failures. Today's seemingly high failure rates are probably due to a combination of issues...some questionable parts here and there, engines have been rebuilt numerous times, low anti wear additives in oils, etc. This all reduces the margin of error.