Yeah, USB drives with their own power supplies work very well with the Pi, but I can imagine high workloads could quickly run into problems and/or bottlenecks.
True, though that bottleneck would mostly be with the Pi itself. After all it only has GbE built-in, a single HD will easily saturate that for sequential loads or all loads for a SSD.
I have 8 disks hooked up to a Pi4 through USB3-SATA[1] with external 12V, works quite well in terms of reliability (had to turn on quirks mode[2] though). As I mentioned the main limitation is the single 1GbE interface, but this is just for archive data so.
Yep, I've also noticed some unreliability when doing things like adding a mirror drive; at some point the USB controller just seems to fall off the horse and everything crashes.
My setup mainly streams video/music over wifi and it's rock solid at lower speeds. I wouldn't rely on it for any heavy duty storage needs though.
I have 8 disks hooked up to a Pi4 through USB3-SATA[1] with external 12V, works quite well in terms of reliability (had to turn on quirks mode[2] though). As I mentioned the main limitation is the single 1GbE interface, but this is just for archive data so.
[1]: https://www.aliexpress.com/item/4000001204302.html
[2]: https://www.raspberrypi.org/forums/viewtopic.php?t=245931