OHC engines are less forgiving, too.
Some bent pushrods in, e.g. a GM V8, are relatively easy to recover from, but camshaft lobes don't bend too easily...
(GM V8s in the '70s often had nylon-coated gears to drive the timing chain - they were quieter than metal-to-metal designs. But if the car ever overheated, the nylon would crack and fall off, leading to chain slippage. If it slipped enough, then ...)
Cheers,
Scott.