The Mathematics of the Manhattan Project

Could it be that the bombs, as their purpose was to be very destructive, required little in the way of safety evaluation other than accidental triggering (not insignificant :) and that they were purportedly very simple devices?

FMEA and safety related strands are a major component of modern day similarly sized projects. Even to the point of preventing them from ever being completed.

An apt metaphor might be the automobile. If it were invented, afresh, today would it be even remotely similar? And what is the ratio of the cost of developing the first viable automobiles compared to the cost of today's relatively minor iterations, per functional unit delivered? Miniscule I imagine.