No, I’m with you, Mark. Considering how often the Galaxy-class ships use their saucer separation (once a year?) it might just be a case of limited screen time. You can always switch things up so I would say in my game “Saucer Separation” is a Talent that only some Nebulas have and I’d allow groups to switch it out if they like.
I agree as well; with how compact the rest of the Nebula frame is, and how the module hangs over the saucer, I can imagine that separating and reattaching the saucer is even more risky than doing so on a Galaxy Class. It just doesn’t make much sense, design-wise, because the slightest misalignment could tear off the module section or damage the deflector dish. At least with a Galaxy class, you have a lot of space separating the Saucer from the other vital parts of the ship–not so with the Nebula.