Well, it's not necessarily cheaper once you factor in the RAID enclosure, especially if doing many drives.
FreeNAS builds are frequently done, as mine is, with a case that holds all the drives and they can directly plug right into the motherboard -- so no potentially expensive external enclosure hardware needed.
That's what I'm getting at.
In any case, let's move on
Both solutions can be great, it's just about how you put it together and what ones needs are.
And yes, TrueNAS/FreeNAS builds can absolutely be done for incredible transcoding performance.