This is going to be a problem for many Watch owners wanting to save its data long term.
Good reason to use iCloud daily and a supplemental (encrypted) iTunes' backup a couple of times a week.
Corrupted backups are rare though. My wife and I actually have backups going all the way back to original iPhone. It's more of a problem for the anal-retentive types who insist on doing clean installs just because.
They are not that rare as I have friends who had bad back ups and I also had a back up that was corrupted. I have always used back-ups to restore my devices but I have had to use a back up previous to my latest due to it being corrupted.
Just because you and your friends have had corrupted backups and my wife and I have never had them are meaningless as they're merely anecdotal. If corrupted backups were common, we'd be hearing a lot more about them right here on MR. The later is a truer indication of how common or rare a particular problem is.
I have read about them here, I did NOT say they were common but for the argument you are trying to change what I said.
If you were to do poll here on MR "Have you ever had a corrupted backup?," the poll results will most likely indicate that it's a rare occurrence. MR tends to attract any mention of problems regardless of how rare or common they are so "having read about them" is meaningless without context. Likewise, more common problems such as the crescent moon camera issue on the iPhone 6/6+ were reported here left and right.
That would mean your comments are as meaningless as mine.