Now, I'm not sure if this is a bug, or just an absurdly unbalancing element, but, in two different games playing races with the "lucky" characteristic, I've found multiple (!) ranger vessels. And I'm talking mid-game, where they can still destroy a good 99% of enemy ships. In the first game, I found two on two consecutive turns (!), then proceeded to utterly destroy all of my enemies (okay, so I'm a little opportunistic...), whilst in the second I found three, though they were more spaced out as to where and when I found them.
So... yeah. This seems a tad crazy to me.