During my testing when placing down on top of a blueprint even though the underground does not spin in hand when placed down it is as per the blueprint.

Not at home at the moment to show proof. If needs be will creat Gif in few hours.

For me it would be most likely that you would place both sides of the underground at any one time. Could only think of a few times that you would only destroy one side (Mainly because you can drive over them). A biter might only attack one half of the belt. But I could not imagine that happens very ...

Hmm good point. Not sure I can convince you either way. As you have pointed out both methods have their own use cases.

Uploaded a GIF.

When placing the first half of an underground belt on a ghost entity it does not spin the 2nd half to connect them up. 2nd half has to be manually rotated.