RAID is inherently less stable/reliable than independent disks. Correct?
In the broadest sense, any extra overhead could be considered a liability. For RAID1, you're adding complexity to ensure uptime. It's a trade-off.

This I don't understand. RAID 1 requires a min. of only 2 disks not 3.
I know RAID1 is only 2 drives. I was trying to understand what you were talking about.

Sounds like you want to synchronize data on multiple drives, but not use RAID. There still has to be some software or other mechanism that syncs the data. That is adding complexity, so it's possible something could get corrupted.

I must go buy a new drive to rebuild the array in order to access my backup data again
That's not necessarily true. You can run on 1 disk with a broken RAID1 array. That's the point of having the RAID1; no downtime. I've done it with servers before. The system will be slower, but still operational. I'm not so sure about that NAS device you are talking about, though, so you probably want to download the manual before you purchase.