There's nothing that's top-level shared. Binary, XML, and JSON are each their own separate paths.
It's just a matter of the volume of work. As long as you support Variant, then the Node, UI, and Component derived types are little effort. However, there's a fair bit going on with techniques, texture XML config, materials, particle effects, etc that will require much more work. Probably quite a bit in Urho2D as well.
It adds up to an awful lot of possible paths to test and confirm.