I tend to disagree with this. In case of an OS failure, you could still
recreate the RAID volume on a new system and restore the data. You
may need to remember the raid configuration to make this happen, but
in most cases (Unix/Linux bases) you can do this.
If you have a hardware based raid, the raid controller failure may mean
you are out of luck if you cannot find a hardware to support what you
had.
For the lower end hardware RAIDs, I tend to get multiple RAID
controllers and keep an extra one as a spare.. just in case if there
is a failure and I can't find a device with similar compatibility and support.
One great solution for creating a raid by yourself is
FreeNAS
Getting a bit off topic, but rather than arguing over software vs. hardware RAID, just pick which one works best for you and then ALWAYS KEEP A BACKUP! Basically, plan for a fire/earthquake/meteor strike/ etc that destroys the entire building your primary systems are in. In that case, it doesn't matter what kind of RAID you're using if you haven't done a backup and moved it somewhere. Never, ever assume that RAID is a replacement for backing up!
Personally, I just use a 500GB external drive, do a backup every few weeks, and keep the backup drive in my office about 10 miles away.
Bob