As I understand his post, there are two big issues.
1. you can't even try to unmount a filesystem if it's mounted under another filesystem that you can't reach
mount /dev/sda /
mount remote:/something on /something
mount /dev/sdb /something/else
now if remote goes down, you have no way of cleanly unmounting /dev/sdb
2. even solving for #1, namespaces cause problems because with namespaces, it is now impossible for any one script to unmount everything, or even to find what pids need to be killed in all the pid namespaces to be able to make a filesystem idle so that is can be unmounted.