There’s a lot to engine design, and something as relatively simple as bearings requires advanced knowledge in tribology and fluid dynamics.

You can design for a certain oil with a certain dilution rate but if the oil standards change or the real-world dilution rate is different, you may have failures due to cavitation. You can design for a certain dynamic clearance but if someone else changes the rod bolt spec or something as “simple” as thread fit changes, you may have failures.

Then there are things like manufacturer errors in fatigue strength or embedability or conformability, things that can only be known with a strict quality control regimen.