I overrode the version requirement to try this out in 2.1. In many hours of play I've come across an intermittent crash when placing undergrounds. Otherwise it's been working great!

I think I was able to make a fix. In control.lua lines 1720 and 1199, replace:

entity.neighbors
with
entity.underground_belt_neighbour

Relevant bit from the 2.1.7 patch notes:

Removed LuaEntity::neighbors read. Added LuaEntity::fluidbox_neighbours, underground_belt_neighbour, wall_neighbours, cliff_neighbours, neighbour_connectable_connections read. Use those and belt_neighbours, heat_neighbours instead.

Hi and thanks for posting this. Hm... I wonder if I might need to pay some more attention to all these other specials for pipes and stuff etc, but we'll see... I'll try to get the mod at least bumped up a version and released in the oncoming days (can't guarantee extensive testing, so anyone using the mod will have to guinea pig it a bit ...)

happy to be a guinea pig if/when you do get it updated for 2.1