Hard drives are a point of failure for all systems. I have had my WorldBook for five+ years now and it keeps chugging along nicely, but I do worry about failure. I decided to switch to a RAID system to backup all my drives to and now have 30TB of networked attached storage, in a RAID configuration to protect the data better. It wasn't 'cheap' to setup, but about $700-$1,000 per unit, with each unit supporting about 5TB.
I have no problems with my WorldBook, I like it, and if it works well for you, then great. But, do remember that hard drives fail and if you have important data, you will want a backup of that data in a good location.