Disadvantage: Questions can be migrated more freely. If Stack Overflow's off-topic popup lists Code Review as a possible destination for migration, that's going to result in many poor questions being migrated on the grounds that "this code is a mess, send it to Code Review to sort out". We don't want to review code that's a mess, we want code that's as good as the coder can make it.
(Counterpoint to this answer by 200_success.)
