I'd imagine a 3D crossfade would be something like standing in front of a shop window on a sunny day, where you can still the street behind you 'semitransparently' overlayed on the shop interior. Once your eye commits to either scene, you can essentially stay focused on it without difficulty. While a 3D crossfade would have changing scene opacities, the only difficult part would possibly be the 50/50 point, which may end up being no more disorienting than a traditional crossfade. The eye would likely stay focused on one scenic element of the old scene until it was too obscured by the new scene, at which point the most likely 'refocus' point would be in the new scene.