Which is why I recommend having more than one backup of your data. I actually swap between 2 Time Machine drives once a week, so in worst case, I only lose a few days of stuff if the TM drive fails.
Where non sensitive data grows but does not change, photos, it would be good to hand out copies to family who live elsewhere. You're then covered for fire and they think you're being nice.
As it stands I've had to restore from TM twice and had no issues. Of course both drives I'm using for TM are both less than 2 years old.
I've done time machine restores and they have worked well from good drives.
I would like to think that drive age has a lot to do with the 'quality of life' of the hardware. Several bad runs of hard disks models has completely cured me of that thinking. Anybody remember the 5.25" IDE Bigfoot drives? I think those had the most reliable expiration date of any drive.