I am not sure where you read that USB is recommended for the OS. I would argue that it’s the exact opposite. It’s not recommended, but it is a better choice than USB-stick, so I guess one could say there are worse choices.
Hardware RAID should be avoided no matter where you choose to install the OS.
If you are not using hardware RAID and your computer fails (but the drives are okay) it’s a simple thing to move the drives to a working chassis and pick up almost where you left off.
If your boot drive fails it’s also not a big deal if you made sure to save your configuration somewhere else, including encryption keys, as you can easily restore your configuration on a new OS install.