[Elyminator]

I just built one out of an old gateway machine with an upgraded PSU and drive cages. I used xpenology. the only real reason not to use ITX is typically they don't have enough sata ports for drives
if it's got enough ports for the amount of drives you want then by all means use it. If youre going to spend hundreds of dollars to build a computer you might as well buy a purpose built NAS..

just my two cents.