I’ve been experimenting with Urho3D terrains lately and, and out of the box, they are pretty limited. Just like the default Unity terrains. There’s a very popular addon for Unity that adds support for multiple materials per terrain mask:
JTippets’ Terrain + editor allow more splat maps than default Terrain.
There have been community contributions on other shader types (geometry).
As for when a particular feature might be in master branch, I cannot say, but maybe another can.