I would call it “1/8 supported then” → you have to build the build from source,
it will have errors and crashes on load… after 2-3 weeks of fighting both UE4 and C++ compiler you will finally get the editor… Thats not how “Supported should be”.
I would agree there are dozen of repos and what not,
but lets agree that Ubuntu is a mainly used one for development.
epic could show some support for atleast Ubuntu