I played online and automatic downloading of all the required resources (including add-ons) did not happen, failed for some reason. Made me think automatic synchronising of all required resources wasn't or wasn't fully implemented.
What resources?
Sound, music, and texture downloading has to be enabled in the network tab of the options menu
Model files can't not be downloaded, the engine can't handle it. if you somehow got into a server with a model displayed that you didn't download, you would be disconnected with the reason "Invalid packet"
Other than scripts, there are no other resources to download, and those won't be downloaded for security reasons as described above