For agent-less protection Rapid Recovery obtains the block mappings from VMware's CBT files in regards to which blocks are active/allocated and which blocks are changed. The information on allocated blocks is not present when it comes to NFS datastores.
From Vmware KB 1020128:
For CBT to identify disk sectors in use with the special "*" change ID, these items are required:
The virtual disk must be located on a VMFS volume, backed by SAN, iSCSI, or local disk.
The virtual machine must have zero (0) snapshots when CBT is enabled, for a clean start.
For customers where identifying allocated disk space on agentless VM's is a requirement, migration to VMFS volumes, backed by SAN, iSCSI, or local disk, is suggested as a possible recourse.
CBT will still be able to track altered disk sectors of VM's on NFS storage.
Also from Vmware KB 1020128
The host must be ESX/ESXi 4.0 or later.
The virtual machine owning the disks to be tracked must be hardware version 7 or later.
I/O operations must go through the ESX/ESXi storage stack. So NFS is supported, as is RDM in virtual compatibility mode, but not RDM in physical compatibility mode. VMFS is supported, whether backed by SAN, iSCSI, or local disk.
CBT must be enabled for the virtual machine (see below).
Virtual machine storage must not be (persistent or non-persistent) independent disk, meaning unaffected by snapshots.