Even a superior species would fall under the Borg, simply because: "So, 2500 Cubes failed? No problem, send in 2.500.000 others..."
I agree that superior numbers can overcome superior technology. But only to a degree. It also depends upon how
superior that species is, 2.500.000 cubes won't do you any good if the other species has the power to simply think them, or the entire borg collective for that matter out of existence (like the douwd did with another race) -- or do it by pushing a simple button, whatever.
Which brings up an interesting question in itself. How would the Douwd or the Q view the Borg ? And conversely, how would the Borg view them ?