This mod is absolutely awesome. However, there are some limitations which I will list below, so you can plan your portable buildings with them in mind.

• Wire connections don't persist. So you might want to save wiring diagram for your fancy airlock or elevator separately somewhere.
  
• Container contents don't persist. Don't accidentally loose your stuff, ey.
  
• Secret doors only somewhat persist. If secret door is closed, it won't be saved as a door, it will be saved just as a bunch of solid blocks. However, if the door is open, it'll be saved just fine.
  
• Crafting stations' upgrades don't persist. It'll save them, just in the most basic version. Something to watch out for when playing with mods that add a lot of them, such as FU.
  
• Liquids don't persist.

That's what I've confirmed so far. I suspect that vehicle-type entities don't persist too, such as rail platforms or, well, vehicles. But I haven't been able to confirm it yet. I'll probably expand this list if I find anything else. You're free to add your discoveries too.