I can't provide a counterpoint for why you might consider a proprietary NAS, but I can tell you that I'm 90% sold on FreeNAS. I've built two servers running them from commodity hardware and never once had a problem with it.
I use rsync to backup to my older NAS no functioning specifically for that purpose. It's a ReadyNAS x86. I moved away from that because I wanted more functionality and redundancy.
I've had 2 drive failures on 5-disk setup. Live rebuild in degraded status worked as well as it should have. I also use the SSD caching feature which sped things up a bit for frequently used files.
My only qualm about the product is that whole versioning fiasco. I actually almost got bit by it while building a server for my parents, but the install wouldn't take so we used the older version. But by the grace of luck we didn't use the bad version.
I use separate hardware for my service host which talks over NAS. I just use the FreeNAS boxen for storage. Learned that lesson!