Yes, yes it is. I have looked after several Raid 5 based business class NAS units for a couple of years. In that time I have have total data loss twice using RAID5 (luckily I had backups).
Based on my experience the following use case is horrible for RAID5 and will result in data loss:
1) Large disks (>1TB) these drives take a long time to rebuild in the event of a failure. Because of the huge amount of data, rebuilding the array depends on being able to read the remaining drives (see number 2)
2) Large number of disks - the more disks in your array the higher the likelihood that a drive will have errors that prevent rebuild from accessing some of the information on the drive
3) Consumer Class Disks drives fail much more often than the more expensive enterprise class drives (not really a RAID 5 specific issue but which drives does your array use?)
4) Archive Data - data such as pictures, video, etc. that is not routinely overwritten and only periodically read. This type of usage does not typically allow the controller or the drive to sense that some sectors are becoming unreliable. The drive sectors can be unreadable and until you attempt to read the data for a rebuild you have no idea. Video streaming is often fairly forgiving and will probably just continue when certain data cannot be read.
It has been my experience with 8TB - 10 drive arrays using SATA drives you will lose data once a year. If you have a choice pay extra for RAID6. If not ensure that you have a copy of the data on another system.
Any number of sources indicate the same for example: http://www.hardwaresecrets.com…-Over-RAID0-and-RAID5/314
As for the Hardware Raid Controllers that someone mentioned, I think I would prefer software RAID like Linux can use. The controllers have smarts built in to sense drive failures and will helpfully take the array offline when that second drive fails. The vendor did not seem to have any help for recovering data but I suspect that there would be a way to convince software raid to keep the array online to recover more data.
regards
Tim