For me personally running Plex server, a single most critical feature would be hardware transcoding support.
Yes, it's mostly limited only to Plex Pass subscribers (I got cheaper Lifetime a while ago), but I think it's invaluable in the sense that you could build a fairly low-power plex server and rely on dedicated hardware to do the EXTREMELY heavily lifting of video encoding.
So this is where I draw a line and unfortunately, BSD is very much behind on this. There are not BSD drivers and this critical (imo) feature will not be supported on any BSD based systems anytime soon.
So it leaves Freenas/Truenas and Nas4Free out of running for plex server. again, according to my needs.
I think Unraid supports hardware transcoding with Plex. So does Linux based OPV (Open media vault -
some tinkering required)
After years of running the Plex server on FreeNAS, I moved it to a cheap NUC with J3455 CPU and it works great. Running Ubuntu 18.04 server. I am still using Freenas but only as a storage system. SMB+NFS.
How to setup Plex with transcoding on Unraid: