I have ZFS running on the FreeNAS server that this setup would replace.

I am really impressed with the Synology package, my existing box has been excellent. Very easy to use for a numbskull like me and if you do loose a drive it seems very good at rebuilding or allowing you to access data while the drive is failing. Worked for me as I didn't loose a thing! I have it double backed up of course, but that isn't the point there

So you think they will be fine used as normal drives then?

Cheers

Cris