A couple of points,
You only need one Genesis devise to act as a deterrent, the tradeoff being leave us alone or billions of your people are going to die in the blink of an eye. You put it on a small ship out in space, nobody knows where, and that is your deterrent.
Secondly, we know they can adapt to the 23rd century. They were able to commandeer two ships, operate them, and even fix them when they sustained battle damage. They can adapt and learn quickly in any environment