Justin Ive found at least a few scenarios where BB assets don't import well without the plugin, is when using the tool "loop cut" then extending out the sections of the model. It seems like the engine REALLY doesn't like when you make weird shapes out of the default meshes.
also, it seems to import models weird if you add a lot of parts together using "merge meshes". It imports things on an angle or out of the tile it's supposed to be sitting on.
If you want, I can give you these models so you can see for yourself what it is about the default import system that hates them so much, just let me know.
