unRAID will disable any disk that has a write error. It doesn't disable a disk for read errors. Often if unRAID can't successfully read a disk after several attempts, it will attempt to write the data back to the disk from the parity calculation, and if that write fails it would be disabled. But otherwise read errors won't disable a disk.
But since you can't monitor the SMART of the disks, you can't get notifications of impending issues with those disks. You do have Notifications setup don't you?
And as mentioned, all drives must work well for parity to be able to reconstruct a disk. You can have a disk problem that doesn't cause it to be disabled, but could still be a problem when trying to reconstruct another disk.