Backup Exec Licensing
As far as I know, Synology doesn't support NDMP, therefore there should be no discussion of the NDMP Agent or NDMP usage with a Synology NAS.
Most of the Synology NASes are on the HCL and I am sure more will be added over time. Listing of a disk device on the HCL is not mandatory to use it and have it supported. As far as I know, all the Symnology NASes run the same DiskStation software, so that shouldn't be an issue.
To back up from a NAS connected via iSCSI requires no Agent licenses as BE sees it as a local disk drive. Only the BE server has access to these files though.
To back up to a NAS connected via iSCSI works fine as BE sees it as a local B2D drive. No Agent license required.
To back up from a NAS via CIFS requires a Remote Agent license, even if the Agent cannot be installed. Be aware that you can generally only do full backups and cannot do proper incrmentals. http://www.symantec.com/docs/TECH68920 This storage can be shared by multiple people/computers.
To back up to a NAS via CIFS works fine as a B2D UNC path, assuming you have credentials working. No Agent License required.
So, iSCSI has multiple advantages, but the big limit is when backing up from an iSCSI NAS is that BE is the only server that can access those files (no sharing like CIFS amongst multiple users/computers).