So to sum it up, Arnold does not support the Falloff and Forest Color maps. This is why some of the assets worked, and some not in my previous posts; the assets simply didn't use any of the two maps.
As for Materials > Using > Automatic Selector, it seems not as critical as the issue above. If I am using Scanline, the Automatic Selector will place assets with Standard-materials. When using Arnold, the Automatic Selector uses Physical Materials.
Arnold supports Standard as well, as long as none of the unsupported maps is used.