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lessdazed comments on Rationality Quotes September 2011 - Less Wrong
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You are quite correct, small pieces of 235U are stable. The difference is that low concentrations of 235U in natural uranium (because of it's faster decay than 238U) make it harder to get to critical mass, even with chemically pure (but not isotopically pure) uranium. IIRC, reactor grade is around 5% 235U, while natural uranium is 0.7%. IIRC, pure natural uranium metal, at least by itself, doesn't have enough 235U to sustain a chain reaction, even in a large mass. (but I vaguely recall that the original reactor experiment with just the right spacing of uranium metal lumps and graphite moderator may have been natural uranium - I need to check this... (short of time right now)) (I'm still not quite sure - Chicago Pile-1 is documented here but the web page described the fuel as "uranium pellets". I think they mean natural uranium, in which case I withdraw my statement that isotope separation is a prerequisite for nuclear power.)
What is natural is something that I, without background other than a history of nuclear weapons class for my history degree, was/am not confident wouldn't vary from solar system to solar system.
The natural reactor ended up with less U235 than normal, decayed uranium because some of the fuel had been spent. I assume that it began with either an unusual concentration of regular uranium (or other configuration of elements that slowed neutrons or otherwise facilitated a reaction) or that the uranium there was unusually rich in 235U. If it was the latter, I don't know the limits for how rich in 235U uranium could be at time of seeding into a planet, but no matter the richness, having small enough pieces would preserve it for future beings. Richness alone wouldn't cause a natural reaction, so to the extent richness can vary, it can make nuclear technology easy.
If the natural reactor had average uranium, and uranium on planets wouldn't be particularly more 235U rich than ours, then nuclear technology's ease would be dependent on life arising quickly relative to ours, but not fantastically so, as you say.