With NAS builds you're usually network constrained. Most of the numbers you see on storage side are gigabytes while network is gigabits. i.e. 1/8th

So if you crunch the numbers you'll see storage (esp flash) is much faster than network, meaning NAS can cut corners on storage. If you're running something like a VM that accesses the storage directly then suddenly storage speed matters

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.