I can only speak for my own experience with a Cyberpower unit that currently
protects my wireless AP, firewall and server. I had disconnection issues with
the UPS when connected to the FreeNAS server in a Master configuration.
Played with it for weeks before moving the cable over to pfSense box as Master
and configuring FreeNAS as Slave. This solved all my issues.
While I was dealing with trying to make it work with FreeNAS, I googled and
read a gaggle of posts and blogs trying to find patterns that would reveal
some kind of consensus as to which model was best for FreeNAS but it was an
exercise in frustration. I responded to your post having only the need to warn
you about Eaton/NUT situation. As for a recommendation, I just don't have
have any advice other than to avoid cheaping out when it comes to buying
both a PSU AND a UPS. IMHO these two components should always be the
best that you can afford.