what prompted me was this question. I feel like it belongs better on Physics.SE
If this is the kind of questions that users would vote for migration to Physics, then that's a reason not to enable user-driven migration there. Users are prone to vote for migration from X to Y when they find that the question is "more like Y than like X". On Math, they often suggest migrating Physics homework dumps to Physics, or personal agony stories to Academia. Neither is appropriate for the destination site.
Migration works when for questions that are written for the correct audience but posted on the wrong site: e.g., a good question about web browser use posted on Web Application, or a good mathematical question posted on Mathematica.
If, however, the question is something of Chemistry and something of Physics... then I'd guess that it's either too broad, too vague, or (best case) should to be rewritten by the author to properly address the audience of physicists. Migration can't fix any of that.
(No opinion on the path to Biology, which is still in beta. I think these two potential paths should be discussed separately.)