Everybody loves a good super-genius, especially one who can construct a super-powerful submarine. Nemo's mastery of science and engineering—as demonstrated by his diving suits and electricity guns, in addition to his sub—is awesome. But it's also alienating. In 20,000 Leagues Under the Sea, Nemo's achievements set him apart from humanity in more ways than one. They mark him as a genius, but they also make him an outcast. His inventions are a century ahead of time and so is he; he doesn't belong in the world into which he was born. Wamp wamp.
Though the Nautilus provides ample evidence of Nemo's genius, it also speaks to his selfishness. His vessel is little more than a cocoon, a comfortable place to rest; odds are, he could be putting his gifts to much better use.
Verne shows us that man is rarely willing to recognize genius in its time. Whatever his particular history may be, Nemo is framed as a victim of ignorance and the fear of innovation.