[JGrossijr]

It's a combination of things, but IMO the biggest factors are lubrication, ramp speed, and valve spring pressure.
Light oils, windage reduction, aggressive ramp speeds, and higher valve spring pressures, all improve performance but reduce reliability. The main culprit of flat tappet failures in Stock was allowing increased valve spring pressure. Careful what you wish or lobby for. The rule of unintended consequences never goes away.