Sharing the entire “Shared” directory is easy a multitude of ways (sym-linking, git submodules, svn externals…) - easily sharing all of the non meta files while keeping the meta files backed up in the project specific repository without setting up each individual file as an external for each project has proven to be very difficult.
Eventually we gave up and now just use a .dll for all of our ‘library’ code.
I tried to use the symbol links to share the Asset and ProjectSettings folder between two projects, which are actually the same project, but with different build targets. The idea was not to wait several minutes every time I change the target. I created the symbol links with the app SymLinker. Opened the project and got an error in Unity editor telling me a temp file could not be opened:
Internal compiler error. See the console log for more information. output was:error CS2011: Unable to open response file: Temp/UnityTempFile-29700b5aa20c2423ba9aa5bd00273061
The file is there. I don’t know what is causing the problem. Does anyone of you?
I can’t speak to the specifics of using sym-links here, but if you are using the same project for different build targets, you should look into the cache server. This is specifically designed to handle the assets with different import settings between build targets.