In Star Trek: Discovery, Georgiou crosses over from the mirror universe, and from what it seems, she will likely stay in our universe.
The mirror universe is probably not a place you'd want to stay in for very long. In one episode of Deep Space Nine Bashir offers mirror O'Brien a chance to cross over for a better life, but he doesn't take it.
With all that in mind, you'd expect a large amount of people attempting to come over trying to escape, but that hasn't been the case. Any canon reason why?