I had a method return type with UnixFDs inside a struct, and wasn't getting the right fd numbers. I debugged the issue and found that the UnixFDIndex fields weren't getting translated by:
https://github.com/godbus/dbus/blob/85c14741/transport_unix.go#L158

I am going to try to work around the issue by flattening my return type, but I figured the bug should be reported also.