I noticed that nodes underground do not snap. e.g. the highway is used with a parallel lane with opposite direction. On Ground everything snaps fine, but after placing a first segment underground the parallel raod would not snap. This behaviour is also reproducible with rails where I tried placing three parallel one-way-railroads underground.
Also there seems to be another issue with underground building since my builder always reported that there is an object in the way, where even the main node gives this message. Which is unusual. This only occured after I placed the three one-way railroads underground. Even a new segment underground can just be placed a few meters long before it runs into this message.

I also use other mods like network anarchy. When activating it I can place them.

All of this works on ground level!