I have never done RAID over USB, but have done various JBOD setups using SCSI. I think the general idea is that USB having such an easily disconnected connector plus the latency overhead on translating SATA to USB to SATA again means you have a higher chance of corruption. SCSI setups typically have connectors with locking mechanisms to prevent easy disconnection.

If eSATA is an option it might be better for the performance and it has a latching mechanism to prevent easy disconnection. You can get a 2-port eSATA PCI card for about 50 bucks.