We have added documentation to explain the migration process and the verifications behind it.
Try the verbose option documented and, if you see any errors, report them in an issue, including the Babel plugins you’re using, and any other additional context. Please report an issue on the Meteor issue tracker.
We’d like to support more setups. Even after several betas and RC, some specific setups may still require specific migrations. If SWC doesn’t work for you, you can keep using Babel by disabling the modern setting (it should fall back automatically unless there’s an unknown side effect). We appreciate your feedback to improve compatibility.