My 421 did over 800 hours whilst I owned it. Yes, the engines needed careful handling, but they never let me down. My normal planned en-route FL was from 200 250, I would aim for 60% power which would give a fuel flow of between 18 20 gall per side with a cruise of 200kts. I had the long range tanks so it gave great range in Europe. Typically I aimed to lean to 60degrees rich of peak on the worst cylinder, the POH says 50 ROP is most economical, 75 is best power so 60 seemed to work well for me.
Take off techniques was always hold on the brakes, slowly roll the power up to 60%, release brakes and continue to feed power on till you reached 100% - no abrupt firewalling of the throttle.
In descent to stop thermal shock worries I aimed to reduce MP by no more than 2 per minute (slow gradual reduction of throttle)
It took a while to get used to the concept of setting the power on approach and controlling descent / speed, with flaps and gear rather than using the throttles. The aim was that once the throttles had been set in approach configuration you tried not to touch them till you flared, aiming to fly the approach at 110kts, bleeding the speed as you went over the fence. I always tried to avoid minimum length runways, again with conservation of my engines in mind.
With the geared props I was always cautious to avoid plug fouling whilst on the ground, using the brakes to control ground speed with about 1200 on the props. Yes, it can be argued that it may have been harsh on the brakes, but they were cheap to get relined compared to a plug change or worse.
You might think all of this was excessive, the 421 was a great plane, comfortable, fast and at 40 gall per hour offered reasonable operating costs provided you respected the engines and avoided the high repair bills which other had experienced. All it took was a little forward thought and planning for the flight profile that you wanted to achieve.
My engines easily got to TBO, care, thought and caution was needed, as was the advice of people who had been operating similar engines for a long time. If your ham fisted or you chase the throttles on approach then its not the engine for you. I fly a Malbu Jetprop now, other than the obvious worry of it only being a single, the handling techniques for approach ect are very similar.