Really what we're talking about is maintaining copies so that if one system goes down, the data is still on a second. My experience is as follows. I may have just been lucky.
I've used (consumer grade) RAID 1 on the server for years. There are two pairs (arrays) of drives, one smaller pair for OS and the larger pair for data. I use a WD 4Tb Sharespace for online backup, and then have single external hard drives for 'final' and offline backup. (I used to use a RAID 1 Zyxel NAS for online backup, but it's got too small) The sharespace is fast becoming the primary repositary, since I'm running out of space on the server, and PATA drives are more expensive now we're all on sata. The current motherboard is about 4 years old with a consumer Highpoint RAID controller onboard.
About 4 years ago 1 maxtor in the server mechanically failed, and as I was off loading data from the one remaining drive to external HDD, the other raid member also failed. Apart from that, I have never actually had an issue with both drives in an array becoming corrupted with virus or anything else.
As regards the power outage... we have infrequent but annoying momentary power outage here in the rural part of the country. As a result, I run the server, network switches, router, and Sharespace off one UPS, and there is a second in the office supplying my workstation and switch there. I have little doubt that a power outage could seriously screw up a whole array!
UPS's are not overly expensive, and only need to provide enough time that the server stops copying to other devices. Beyond that (after the server has shut down), it seems that most raid devices can just go down ungracefully without any losses. I might be wrong here, but I've never had a problem with the Zyxel in this respect.
At the end of the day, you can only do so much. I try to do what is reasonable. If the server hard drives, the Sharespace and external hard drives all fail simultaneously, I'll just have to shrug my shoulders and think of it as an opportunity to develop a new portfolio!