This lacks some in-depth analysis.
What is the expected support level for this solution? Self-service like a home user? Professional admin? What are the performance expectations of the customer? What is the budget? Is there a power consumption ceiling?
For my customers I've been through (most) NAS solutions and they are all underpowered home toys - apart from those which are
actually servers in disguise. They are the most expensive, though.
If they want to administer the file server themselves, a NAS seems a logical way to go, nevertheless all those "SOHO" boxes are slow, especially in processing numerous concurrent file accesses. Ease of administration is a pro here, buggy software and mediocre performance the major cons.
I personally always suggest setting up a real server (have great experience with Dell R710 series). I buy them refurbished, with warranty, prepared by the refurbisher with latest BIOS/firmware etc. Load them with 8TB drives, hardware RAID10 'em. From there on, there are two routes:
- go full command line ninja and install a Linux (I prefer Ubuntu) with SMB. Pros: oh, the control, oh the flexibility. Cons: tricky to administer (but if your client really wants to do it by himself, a limited Webmin account will do), lots of unnecessary stuff to bother for a fileserver.
- go midway with TrueNAS - install it as the OS and it's fairly easy to administer (about as hard as a SOHO NAS).
Worth mentioning: I never install on bare metal, free ESXi first and virtual machines on top of it.