Yes, mostly. PCIE 3.0 x1 lane gets you 500mb/s [1] not much point to it compared to a regular ssd on sata3 which could do 600mb/s. So if you want very fast storage you would also think about it.

Realistically for your DIY stuff you're talking a different beast to these NASs. Bang for your buck you'd be attaching the mass 3.5" storage in an external second hand JBOD enclosure and the main device would be dealing with the faster storage and have an HBA to connect to it.

[1] edit: as Havoc pointed out I need my coffee should be 2gb/s which does change the point.