iSCSI is a SAN technology, not NAS.
SAN protocols (iSCSI, FCP) are fundamentally different than NAS protocols (NFS, CIFS, SMB).
The difference is simple and important, NAS works at the file level, SAN works at the disk block level.
With a SAN, a host operating system manages all the underlying mapping of disk space allocations that make up a file. Just the same as DAS does. SAN therefore requires a host to make sense of the storage.
With a NAS, the NAS itself manages the storage and presents its assets to the network as file shares which can be accessed without having any knowledge of the way the file is actually stored. It requires no external host.
SAN's allow the host OS to use the physical drive information to retrieve and store data more efficiently than NAS. With a NAS, the OS can only request logical file segments with out regard their implementation. This double translation of logical/physical mapping is one of the reasons for the reduced performance seen in NASes.
NAS's are easier to setup, and work with almost all OSes. Truly plug and play. They are best when sharing files with many hosts.
SAN's have to be thoughtfully considered, depending on what hosts and what OS's are to be used. They are best when sharing with a limited number hosts needing high thruput.
As you might imagine, backup and archive strategies are also impacted by this choice.
Now, to be sure, some NAS's can connect to a SAN for additional storage. Still they only present the file level abstraction to their clients.
Also, some hybrid boxes, i.e. NAS/SAN boxes, can present both NAS drives and SAN targets. This does not mean that the iSCSI portion is a NAS though, anymore than an IP/analog camera is a NTSC network camera.
The thing is, if the protocol between the host and the storage is iSCSI or FCP then the caveats for SAN's apply. If the protocol is NFS, CIFS or SMB, then the NAS considerations apply.