First of all - setting iSCSI both from Linux or WIndows is just an easy task. Maybe even little easier in WIndows (GUI) but under Linux console it is just 2 or 3 lines of commands.
- stable (more stable than NFS/CIFS/SMB)
- if multipath is selected you can benefit from stacking network interfaces
- that is universal solution which isn't affected by CIFS/SAMBA limitations (locks, buffers size, aio, SMB protocol version and implementation)
- you can use filesystem of your choice (NTFS/REFS) or in Linux - ext3/4/Reiserfs/btrfs/xfs/zfs) - with all its perks
- depending on 'what is under the hood' ISCSI implementation on you NAS may be buggy or unstable
- it is recommended to separate storage traffic from regular network traffic (so different vlans at least, new network equipment recommended)
- mostly, your NAS requires you to choose: file or block iscsi version. Trick is, that some of it's features works ONLY with one of them (ODX in Synology for example)
- it is not so easy to reach and grab files from that volume. It is not impossible (mount read only on other workstation/server) but requires more work than quick mount SAMBA?CIFS folder.
- filesystem on filesystem sometimes is more vulnerable to damage
- there is a little overhead in using file mode in ISCSI (filesystem on filesystem)