It is mostly related to the zone of comfort for stereoscopic viewing, which is restrained by the vergence-accommodation conflict. When viewing stereoscopic content our eyes focus on the screen plane but they converge on a virtual point at a different distance. This creates a conflict that doesn't exist in real life, hence producing visual discomfort and fatigue.
For out of the screen effects in cinema, the corresponding value for disparity between left and right images has been found to be somewhat similar from both stereographers experience (2-3% of screen width) and recent research in this field (3-4% of screen width). That's the reason why recent movies don't use many out of the screen effects, or for short periods only.
You can find a complete study about these numbers in this recent publication :http://www.journalofvision.org/content/11/8/11.full
Out of the screen effects also tend to break the suspension of disbelief, which shouldn't be a problem in a ride movie but could break immersion in a feature film depending on how it's used.