!!!OLD WIVES TALE ALERT!!!
***In geared 375 hp continentals, rapid power reduction = death. Gear boxes will not tolerate being driven backwards...***
It's the gear slap that is the problem and that observation holds a lot of truth.
***and engines need to be ROP with easy power reductions due to marked inability to match fuel flow to air cooling flow (despite lots of aftermarket injector money spent).***
Confucius say, "Do not tell man something impossible when he already do it!"
I've run C-421s at 35"/1800 and 50-60dF LOP with no problems whatsoever. (Yes, the top of the green arc on MP is 32.5", but that's set in the event one runs it ROP. When LOP, the MP can be higher with less stress on the engine. The f:A ratios were well balanced with GAMIjectors and a LOT of fixing induction leaks for which these engines are known---and the engines ran beautifully--to TBO. Those power setting result in the same TAS as the normally used ROP MP and RPM, BUT with lower CHTs, and on 17 gph rather than 21 GPH per engine.
The GTSIO-520 was developed under the watchful eye of Carl Goulet who was at the time the VP of engineering at TCM. I recall flying those engines as described above with an FAA DER on board who said, "This is how God and Carl Goulet INTENDED these engines to be run."
Be careful in assigning causality to a valid observation.