It seems to work intermittantly/sketch-ily when there's a lot of folders/sub-folders. Initially, I had the UPnP share out the Root, but it never worked, so I changed it to share out just my videos folder, and it works fine, now.
Now, working "fine," means it works about, oh, a fraction as well as XBMC running on my Xbox 1, but that's to be expected ;-)