I was more thinking about the risk of locking not working as intended on NFS (http://0pointer.de/blog/projects/locking2. See also the section on Nightmare File System in the UNIX-Haters Handbook (https://en.wikipedia.org/wiki/The_Unix-Haters_Handbook)).

There are workarounds for the stateless mess of NFS in modern linuxes, but if you accidentally access a NFS-mounted database from multiple AWS instances, you might get into big trouble, especially when the machines in question run different OSes (e.g. Windows and Linux) or OS versions.