At present if you have deployed Shared iPad, the memory space occupied by the cached shared users remains in use on device, any attempt to update the OS actually results in an out of memory issue.
At present the system does not report or handle this in a sensible way that follows the natural workflow. You can visit every device page and check the memory for each device but that is not efficient.
A couple of feature requests come out of this:
1) provide the amount of memory available on device in any OS Update page so you can see whether it is possible that this can't be installed because of memory issues
2) Intelligently remove cached users that have been written back to iCloud storage when an OS update is in play