Assuming the pool is redundant when a device fails/drops offline the other one continues to work, it's important to monitor the pool to act as soon as possible, if the device failed you can replace it to rebuild the mirror, if it dropped offline you can bring it online then run a scrub to put it back in sync, note that btrfs can only repair the data if COW is enable, for any shares with COW disable, and this was the default for the system and domain shares before 6.10.0, it won't be able to sync the dropped device to due NOCOW also disabling data checksums.
There are some corner cases, especially when a device drops and comes back online that can cause some issues, but that can usually be solved, and that's also why it's important to monitor the pools, to minimize that risk.