As a warning before i get into this topic, yes i know the campaign was only made to be compatable with l4d1, versus is still not done, and neither is the finale.
Does it work? Yes
Missing textures? A ton (textures from l4d1, all/most custom textures work)
events not going off? No events work fine.
Missing models? A ton (models from l4d1, all/most custom models work)
As i said it is playable, works just fine if you can ignore the missing textures and wireframes.
My only main issue with the compatibility comes with the calling of L4d1's uncommon common. Aka the military common, hospital, etc. They move like zombies, sound like zombies, but look and collide with the player models like giant ERROR signs. If you are not carefull though you can get trapped by them. You can croutch jump on them, and walk on them, but if you kill 1 in a doorframe SOL, unless you can noclip.
Weapons, ammo, health kits, pills,etc spawn just fine. Additionally baseball bats do spawn in the safe rooms.
Not sure if anyone else tried it and I just wanted to share my findings. Maybe i can get a youtube video of it later.











