Have you looked at the min specs for freenas lately? Thats a fairly good computer just to sit around and wait. Might as well use a xserve and have no compatibility issues. I found info a long time ago about the xserve g4 beeing able to handle 2tb drives with sata adapter 4x2tb is a nice storage pool to use, pair that with a xraid 1tb each with sata adapter that adds 14tb to it and you dont have to deal with 8gb ram minimum and zfs and such at all.
the specs were not that extreme when i first brought it up back in 2014.. the min specs were greatly increased between 2014 + 2017/2018
if you have an older version of freenas/nas4free like the one i use.. u can still do it with a machine with 1gb ram etc
you could easily do it on an old intel core or pentium 4 with a case that can take 4,6 or 8 hard drives
i have 2 seperate servers that i use for double redundancy
not everyone has an xserve.. i dont. PC hardware is widely accessible + available. and sitting around waiting to find such a usefull use.
XSERVE cant hold a candle to the performance that i get from my sata3 fileservers. that i can promise you.
why compare performance between a machine from early 2000s to one fitted with hard drives from modern late 2010s?
especially when a sata3 adapter is 25$ and you can buy sata3 2tb hard drives for 60 bucks