Same setup here, two USB drives dangling from my NUC. One of them is even notably slow for a USB drive. Still not an issue at all for home use. I’d probably need a dozen or more people all watching different things on Jellyfin at the same time before it even approached being a problem.

The difference between building your own car and designing your own internal combustion engine.

That tracks with my experience as well. Literally every single Seagate drive I’ve owned has died, while I have decade old WDs that are still trucking along with zero errors. I decided a while back that I was never touching Seagate again.

I wasn’t speaking about PPPoE specifically when I made my post, all wired ethernet traffic only travels from sender to recipient without being visible to any other devices that’s not in the direct communication chain. This wasn’t always true. A network hub will send out incoming data to every single port, but hubs haven’t been in common use for decades. A network switch is aware of what is plugged in where, and will only send received data out whichever specific port the destination is connected to. If you have three PCs plugged into a network switch and PC1 needs to send a packet to PC2, PC3 has no way of even knowing it happened.

That said, your final point is correct, and ARP spoofing defeats this. It had completely slipped my mind when I made the above post.

accessible to any device on the LAN.

Only if that traffic is using broadcasts. Wired networking on moden hardware is strictly point-to-point, PC1 is completely unaware of any traffic between PC2 and your home server or whatever.

Wireless is different and can ostensibly be snooped by anything that knows your network key, but I’d assume that you’re not running services on wireless devices.

There isn’t much difference at all. Neither should have a cap.

Data moving across a network doesn’t have any per-unit cost to the people operating the network. Whether you use 5TB or 5GB doesn’t impact the bottom line of the ISPs at all.

The only justification for a data cap would be if they’ve overprovisioned their network and sold too many people plans that are too fast for their network to support, so they need to disincentivise people from actually using it. Even then that’s pretty shaky justification.