Can corruption happen by powering off the computer (not properly shutting it down) during balance or scrub of a btrfs volume?

Since btrfs uses Copy on Write, as long as the data makes it onto disk in tact, any further btrfs operations on the data will be safe against sudden power loss. It might need the opportunity to repair some stuff once power is restored (scrub), but the data (and metadata) should still be there and recoverable, not left in some partial state that can't be resolved.