/usr/share/nginx is the default root path for nginx. It sounds like Ubuntu has just stopped trying.
At least they didn't locate it on the root directory. Mount points should never be on root. See the sources for stat() for the main reason why not.
/boot is typically located on the same disk as root, so that should be ok. /srv and /home, however, would be problematic root mounts. You can use automount to fix /home but /srv should be mounted under /usr or /var.
Mounting NFS or busy disks on the root dir is not a good idea on any server where filesystem i/o performance is an issue.