Just say no to Grey Goo....
...regardless of the scale. Even macroscale replicators could concievably have a transcription error and cause a galactic mess with benign tribbles or more virulent "alien killing machines" rampaging through the local spiral. Then the survivors looking for retribution.....a mess indeed.
Speaking of SF it's kinda like Bear's two book series.
BTW, I love how the result of Fermi's lunchtime chat with other Manhattan Project luminaries is called "The Fermi Paradox" with such signifigance attributed to it.
As if it were accompanied by a lifetime of research and pondering....