I would suggest that these kinds of comments should always be phrased like the following:
Your question is off-topic here, but on-topic for xyz.SE. However, please do not re-post on the other site, your question will be migrated there automatically.
I voted to migrate your question to xyz.SE. It should be migrated shortly.
This will however need one of the following:
- A possible migration path for close voters
- The person writing the comment flagging the question for moderator attention to have it migrated
- Confidence that the question is really off-topic here and on-topic on the other site.
This means for every actively moderating community member that they should always flag a question where they drop such a comment on, or vote to close appropriately, if and only if they are confident about what they're doing.
Comments without further explanation for the OP should IMO be deleted to avoid confusion. Eventually, a question will find its way to the site it fits best on.