The bitrot scenario is not when a drive fails or bad sectors, its when a storage bit is changed.
The real question of bitrot is whether bits on the storage media can unintentionally and unexpectedly change (silent error)?
What's the hardware going to do?
What is the filesystem going to do (btfrs)?
What is Unraid going to do?
It is my understanding both btfrs and SnapRAID can handle bitrot/silent errors (though SnapRAID is faster and higher quality checksums).