> If something works "automagically" like this then I jump into thinking about failure modes. Will it mount if one of the other devices are missing or failing? Will it just warn that the raid is degraded and I should add a new device? Will it mount read-only?

That's all well-documented, but it can also be summarized by saying that btrfs does the right thing. It won't mount normally if it's missing any devices, because running in degraded mode shouldn't be a silent error (especially at mount time).