I have a 1974 P30 and "had" an Atomic 4 which was totally unreliable at the worst possible times. I had it swapped out for a M-25XP Westerbeke Diesel and have never regretted the decision.
That's all well-and-good, and I'm happy it worked out for you, but, by all accounts: The venerable old Atomic 4 might just be the quietest, smoothest-running, most reliable power plant anybody's ever put in a sailboat.
However: They are not perfect. They are not trouble-free. Some are better than others. Sometimes they break. Such is the nature of anything created by Man.
Nice rant, BLJ. Bravo!