I definitely have not tested everything, least of all actual multiplayer because my local server hosting is busted.

- Clone survivors are not targeted by witches.
Valve, the stupid dum-dums they were, decided to always use the first survivor that matches the character for the witch to go after. They also do this for other things like Defibs and chargers charging into people.
Examples:
Nick startled the witch but she goes after you as Bill in L4D1 maps.
You have 4 Coach players. Whoever startles the witch will make the witch go after the first Coach, who is usually the local host or first joiner of the server.

- Finale rescue cinematic and the credits don't account for the extra survivors.
Another map-based hijink causing the extra survivors to be left behind in the ending cinematic. They will also show up as dead in the credits. Fixed.

- Maps that use L4D1 survivors as NPCs will incorrectly use the playable L4D1 survivors.
Many maps don't check if the L4D1 survivors they have are playable at any time, so they can get locked into position, deleted, and generally just be screwed with a lot, and possibly even soft-lock the map.
The Passing is an example.
Fixed.

L4D1-survivor-using maps are notorious for completely erasing the L4D2 survivors from use, forcing me to FAKE them. At this stage of the mod, the fakes are not yet fully supported.
- The L4D2 survivors are still treated by the game as clones of the L4D1 survivors.
Everyone will name them as if they're L4D1 survivors.
This is one issue that is hard to fix, as I have to manually manipulate and consider everything the talker does.
It also means every issue that applies to clone survivors also applies to these fakes as well.

- The L4D2 survivors use L4D1 names, arms and icons.
The survivor set information that says which arms and icons to use is completely client-side and determined by the mission text file for your machine.
You'd have to manually edit the mission text files to use the 2nd survivor set where everything works properly, and that is absolutely not feasible for this mod.

- L4D2 survivors sometimes use L4D1 survivor voices
Not sure how this manages to break, but I've seen it happen trying 18 Coaches.
Possibly fixed.

- Cannot use jointeam 2 nick or Talker Join mods to play as L4D2 survivors
Related to clone survivors being unnatural in the game's code.
Unfixable. Workaround is to use !survswap instead.