Belt Balancer 2 (2.0.9) doesn't load on Factorio 2.1 because of factorio_version. Raising that to 2.1 in info.json makes it load, but rotating an underground belt would then crash, because 2.1 removed LuaEntity.neighbours. The fix is one line in control.lua (around line 183, in the on_player_rotated_entity handler):
-- before
local neighbour = e.entity.neighbours
-- after
local neighbour = e.entity.underground_belt_neighbour
With that and "factorio_version": "2.1", it runs for me on 2.1.21: a save with about 90 balancer parts loads and runs without script errors. I haven't tested every interaction in play.
The GitHub repo is archived, so I'm posting it here for the author and for anyone patching the mod locally.
