Flyable and non-flyable FM differences etc.

[The Stalker]

I was always curious about this so might as well ask here.

Can someone explain the differences in the modeling of flyable and non-flyable craft in the sim, regarding the performance and behavior in flight. Are the ones that are flyable modeled in more detail, eg. their behavior in flight, performance, speed, stall, climb characteristics than the ones that are AI only.

I'm asking this because mods like UP for example make nearly *all* craft flyable, so I would like to know where are they "cutting corners" when they do this.
Obviously the first thing that comes to mind is that the cockpits are probably not very accurately modeled, but I wonder what else is suffering from such conversions of AI only to flyable?

So basically what are the differences in FMs of flyable and AI only craft, besides the cockpit interiors?