It also has an M.2 slot for the I/O heavy stuff. There is a natural limit to how fast you want to stream movies from disk, so I wouldn't care. Its just that for the same price, I can get a Synology with a 4C Celeron which wouldn't be much worse at far less effort.
It also has a PCIe x4 Gen 3 slot that you could drop a SATA card in if you really want to go big.
A 10 second search turns up plenty of options that will turn that port into 10+ SATA ports for like $65. Dunno about the driver situation in that case though.
The single M.2 slot would probably be used for the OS and maybe a cache. Having just 1 fast drive in an array usually isn't a win.
