I agree that landfills should considerably change the eco footprint and I also think this feature is quite unbalanced.
You can see if and how much an object impacts the eco footprint in the BB catalog. For example the standard BG sidewalk slabs floor tile (unfortunately one I really like to use because it's so neutral aesthetic wise) really changes the footprint, especially if you use it a lot on large lots. I once built a highschool using this tile (and another one with eco footprint industrial as a roof tile on the flat roof) and the lot was extremely smoggy to a point that it was really ridiculous... Maybe that's a workaround for you.
