They insist it is the most immersive world in a computer game, ever. Most of my uses of 'immersive' don't seem to apply to U9, and I certainly wasn't immersed when I played it. So let's turn to Webster for a real comparison. One answer is atmospherics. The Dark Project is an excellent example of immersion via atmospherics.
Suspension of disbelief is a very tricky thing to get right, yet some games do it quite well.
Someone in the front row stands up, blocks part of the screen, and burps really loudly. You're not immersed anymore. You're just annoyed. Yes, I patched the game before I played it through. There are a lot of bugs still present, mostly annoyances that simply ruin the suspension of disbelief.