@minkiu For some uses: disk I/O is slow, over USB, and RAM is too small to compensate that with caching. The amount of max disk space may be too limited.
I've run a NAS with all disk space provided over USB, and it was not a reliable experience, but that was many years go. Maybe USB3 disks are more reliable.
However, that may not apply to your use case. I'd not hesitate to try, given the price, if my NAS needs were much more modest than they are. If you have the time to try, go for it.
Lars and friends