At least for RAID1 and RAID10, you should use two channels (and depending on what is doing the
mirroring, two controllers), and two enclosures. Each half of a mirror pair is on a separate
enclosure and channel.
An electrical failure can still knock out the entire system, as the buses are not optical, but
the usual failure modes for drives nowadays have them at maximum locking up the SCSI bus (and
usually they don't get that bad). Shorts are not common IME.
Anyway, with SAS and SATA, which are point-to-point links, the use of separate enclosures (due
to the power feed) and controllers becomes the real point.
One point of notice: Linux is not good for reliability here, unless you can remove the "IRQ
handler killer" from the picture (e.g by using extremely good controllers that don't do stupid
things, and only one device per IRQ line). You have been warned. A lot of failure modes
cause weirdness on the controllers, and may cause spurious IRQs. If the kernel kicks the IRQ
line away, there goes everything tied to that IRQ line. This is specially bad on SATA.