I agree, there's always room for doubt. That's the true foundation of science: keeping an open mind.

Many cities in Europe and Japan were leveled by saturation incendiary bombing, a truly heinous war crime (Kurt Vonnegut's Slaughterhouse Five is based in part on Vonnegut's actual survival of the Dresden firebombing; he was a war prisoner working in an underground slaughterhouse when the air raid began).

I don't think it likely that such a raid caused the destruction of Hiroshima and Nagasaki; such raids involved multiple waves of many, many bombers. But who knows . . .

Micro-nukes are alleged to have been used (along with thermite on the upper floors) to bring down the Twin Towers; the evidence I've seen seems credible. Beyond that possibility, why haven't we had terrorist (or government, or accidental) incidents with nukes of whatever size? Just lucky? Good work by those tasked with preventing such events? Aliens helping us out?

I have no idea.